I  mini  ill  mill  nil'1"11"111111" 
B    3    375 


-IE 
3ITY 


SUPPLEMENTARY    INVESTIGATIONS 

OF 

INFRA-RED  SPECTRA 


Part     V — INFRA-RED  REFLECTION  SPECTRA 
Part    VI — INFRA-RED  TRANSMISSION  SPECTRA 
Part  VII — INFRA-RED  EMISSION  SPECTRA 


WILLIAM   WCOBLENTZ 


WASHINGTON,  D.  C. 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 

1908 


SUPPLEMENTARY    INVESTIGATIONS 

OF 

INFRA-RED   SPECTRA 


Part      V — INFRA-RED   REFLECTION  SPECTRA 
Part    VI — INFRA-RED  TRANSMISSION  SPECTRA 
Part  VII — INFRA-RED  EMISSION  SPECTRA 


BY 

WILLIAM   W.lcOBLENTZ 


WASHINGTON,  D.  C. 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 

1908 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
PUBLICATION  No.  97 

PHYSICS  UBRARY 


The  Plimpton  Press  Norwood  Mass.  US. A 


PHYSICS 
v  MBRARY 


CONTENTS. 


PART  V.  —  INFRA-RED  REFLECTION  SPECTRA. 

Page 

Chapter  I    9 

Introduction    9 

Chapter  II.  —  Infra-red  reflection  spectra  of  various  substances 10-20 

Carbonates  > 10-13 

Sulphides I3~I5 

Oxides   15-17 

Aluminum  silicates    18-20 

Chapter  III.  —  Minute  examination  of  the  reflection  bands  of  quartz  and  of  the 

carbonates 21-24 

Chapter  IV.  —  Reflection  spectra  in  the  extreme  infra-red 25-34 

Apparatus  and  methods 26-28 

Residual  rays  from  mica    29 

Residual  rays  from  carbonates    29-31 

Residual  rays  from  cryolite   3I~3 2 

Residual  rays  from  other  minerals  32-34 

Chapter  V.  • —  On  regular  and  diffuse  reflection 35~38 

PART  VI.  —  INFRA-RED  TRANSMISSION  SPECTRA. 

Chapter  I  41-58 

Introduction  41 

Group  I:  Transmission  spectra  of  various  solids 4i~47 

Group  II:  Transmission  spectra  of  various  solutions 47-51 

Group  III:  Transmission  spectra  of  colloidal  metals 5i~53 

Group  IV:  Transmission  spectra  of  colored  glasses  54~58 

Chapter  II.  —  Effect  of  special  groups  of  atoms  on  radiant  energy 59-68 

Comparison  of  ultra-violet  and  infra-red 59 

The  hydroxyl  group 59~6o 

The  effect  of  molecular  weight  of  the  maxima  60-64 

Characteristic  bands  of  quartz  and  of  silicates 65-68 

PART  VII.  —  INFRA-RED  EMISSION  SPECTRA. 

Chapter  I    71-92 

Emission  spectra  of  metals  in  hydrogen    7i~72 

(a)  Nickel  arc  in  hydrogen   71 

(6)  Spark  spectra  of  metals  in  hydrogen 72 

Emission  spectrum  of  the  carbon  arc 73~78 

The  investigations  of  Moll 76-78 

Radiation  at  room  temperature 78-81 

Selective  radiation  from  the  Nernst  glower 81-87 

Radiation  from  metal  filaments , 88-92 

3 


4  CONTENTS. 

PART  VII.  —  INFRA-RED  EMISSION  SPECTRA.  —  Continued.  pa 

Chapter  II.  —  Selective  radiation  from  various  solids    93-132 

Introduction 93~Q6 

Radiation  from  electrically  heated  solids 96-1 1 1 

Emission  spectra  of  solids  on  Nernst  "heater  tube" 111-124 

Relation  between  emissivity  and  energy  consumption 124-1 29 

Summary    ^ 1 29-132 

Chapter  III.  —  Radiation  from  selectively  reflecting  bodies,  with  special  reference  to 

the  moon 133-146 

The  effective  temperature  of  the  sun I33~I3S 

The  limiting  temperature  of  the  surface  of  the  moon 135 

Fall  of  temperature  of  the  moon  during  eclipse 136-137 

Reflection  and  radiation  from  the  moon 138-139 

Emission  from  a  partial  radiator 140-146 

Appendix  I:  The  effect  of  the  surrounding  medium  upon  the  emissivity  of  a  sub- 
stance      147-151 

Appendix  II:  Instruments  and  methods  used  in  radiometry 152-176 

I.  The  microradiometer 153 

II.  The  radiomicrometer IS3~I55 

III.  The  thermopile 155-158 

Comparison  of  old  and  new  form  of  thermopile 156-157 

The  Peltier  effect 157-158 

IV.  The  radiometer    158-165 

Comparison  of  sensitiveness  and  area  of  vane 159-160 

Comparison  of  sensitiveness  and  diameter  of  fiber  suspension 160-162 

Sensitiveness  compared  with  wave-length  of  exciting  source    162-163 

The  radiometer  compared  with  the  bolometer 163-165 

V.  The  bolometer  with  its  auxiliary  galvanometer 165-175 

Historical 165-169 

Comparison  of  sensitiveness  of  various  bolometer-galvanometer  combi- 
nations   170 

Comparison  of  a  bolometer  with  a  thermopile   170-172 

Experiment  with  a  sectored  disk 172-175 

VI.  Summary     175-176 

Appendix  III:  Additional  data  on  selective  reflection  as  a  function  of  the  atomic 

weight  of  the  base 177-180 

Addendum 181-182 

Index  of  substances 183 


PREFATORY    NOTE. 

The  present  volume  contains  supplementary  data  on  doubtful  points, 
which  arose  in  connection  with  the  preceding  work  on  infra-red  spectra. 
The  various  phases  treated,  except  Part  VII,  Chapter  I,  (4)  and  (5),  and 
Chapter  II,  were  ready  for  publication  during  the  summer  of  1907,  when 
through  the  generosity  of  the  Carnegie  Institution  of  Washington  it  was 
made  possible  to  combine  all  the  material  into  a  new  volume,  which  was 
then  delayed  for  the  additional  data  on  emission  spectra  of  metal  filaments 
and  insulators,  thus  rounding  up  the  subject  as  completely  as  is  possible 
at  this  time.  This  completes,  for  the  present,  the  "program  of  investiga- 
tion." The  subject,  however,  is  not  exhausted  —  not  even  thoroughly 
initiated  —  for  we  know  but  little  more  of  the  cause  of  absorption  and 
of  emission  lines  and  bands,  other  than  those  due  to  several  well-known 
groups  of  atoms,  than  we  did  twenty  years  ago,  when  Julius  and  Angstrom, 
independently,  examined  infra-red  spectra.  Each  renewed  effort  is  a  step 
in  advance,  as  the  present  data  on  the  effect  of  molecular  weight  illustrate. 
The  main  problem  is  to  obtain  suitable  material,  which  seems  to  be  partly 
a  matter  of  chance.  For  example,  one  of  the  first  substances  ever 
examined  for  selective  reflection  was  quartz.  It  is  the  easiest  obtainable, 
and  it  illustrates  the  question  of  selective  reflection  better  than  any  other 
substance  yet  found,  save  carborundum,  which  was  one  of  the  latest  min- 
erals to  be  examined.  On  the  other  hand,  after  examining  over  300  dif- 
ferent substances,  it  has  remained  until  the  very  last  to  find  a  series  that 
so  well  illustrates  the  effect  of  molecular  weight  as  the  data  on  the  car- 
bonates herewith  presented.  The  reflection  spectra  of  the  carbonates  and 
nitrates  in  solution  deserve  further  study.  They  are  easily  obtainable,  and 
their  reflection  bands,  which  are  strong,  lie  in  the  region  of  the  spectrum 
where  the  radiation  is  quite  intense,  so  that  no  serious  difficulty  need  be 
anticipated.  Colloidal  metals  also  deserve  further  attention.  Artificial 
substances,  except  carborundum,  have  never  been  examined,  and  it  is 
intended  to  make  a  study  of  the  silicides,  provided  they  can  be  melted 
into  homogeneous  masses. 

In  Part  III  (Carnegie  Publication  No.  65)  the  discussion  of  the  accu- 
racy attainable  refers  to  the  region  up  to  about  12  /*.  Beyond  this  point 
the  absorption  of  the  rock-salt  prism  increases  very  rapidly,  and  it  was 
not  possible  to  attain  great  accuracy.  In  fact,  the  writer  has  given  but 
little  weight  to  his  isolated  observations  lying  beyond  14  /*,  although  others 
have  been  able  to  verify  them,  a  notable  example  being  calcite  (Carnegie 

5 


6  PREFATORY    NOTE. 

Publication  No.  65,  p.  70),  where  the  probability  of  a  band  at  14  JJL  is 
based  upon  two  spectrometer  settings  and  only  three  observations. 

In  Carnegie  Publication  No.  65,  Appendix  V,  "Note  on  Blowing 
Quartz  Fibers,"  the  originator  of  the  method  was  then  unknown  to  the 
writer.  Since  then  it  has  been  found  that  in  addition  to  shooting  quartz 
fibers  the  method  of  blowing  them  is  fully  described  by  Boys  in  the  London 
Electrician,  p.  220,  Dec.  n,  1896,  "Blowing  and  Shooting  Quartz  Fibers." 

Prof.  E.  F.  Nichols  has  written  me  that  he  found  the  method  in  1891, 
and  exhibited  it  at  the  jubilee  celebration  of  the  Physikalische  Gesellschaft 
in  Berlin  in  1896. 

In  Carnegie  Publication  No.  35,  p.  51,  line  10  from  the  bottom  should 
read:  "Carbon  dioxide  is  the  only  gas  studied  which  has  no  strong  absorp- 
tion bands  except  at  4.5  /*  and  14  /*." 

W.    W.    COBLENTZ. 
WASHINGTON,  D.  C.,  May,  1908. 


PART  V. 


INFRA-RED  REFLECTION  SPECTRA. 


1CHAPTER   I. 

INTRODUCTION. 

In  the  preceding  volume,  Part  IV,  the  reflection  spectra  of  several 
groups  of  chemically  related  minerals  were  examined,  including  sulphates, 
sulphides,  and  an  extensive  group  of  silicates.  This  examination  did  not 
include  the  oxides  and  carbonates  which  were  not  obtainable  at  that  time. 
To  examine  the  substances  in  chemically  related  groups  seemed  to  be  the 
only  logical  way  to  gain  insight  into  the  mechanism  of  selective  absorption 
and  of  selective  reflection.  The  main  difficulty  lay  in  obtaining  material 
of  sufficient  size  to  produce  a  suitable  reflecting  surface.  A  list  of  minerals, 
which  occur  in  sufficient  size  and  homogeneity,  was  sent  out  to  various 
mineral  dealers  who  very  kindly  submitted  several  large  boxes  of  specimens 
for  selection.  Even  after  this  first  elimination  it  was  possible  to  obtain 
only  about  10  per  cent  of  the  number  of  minerals  desired.  In  addition  to 
these  minerals  a  considerable  number  of  specimens  were  selected  from  the 
collection  in  the  U.  S.  National  Museum. 

The  apparatus  and  methods  employed  were  essentially  the  same  as  in 
the  preceding  investigation.  The  spectrum  was  produced  by  means  of  a 
rock-salt  prism,  and  mirror  spectrometer  previously  described.  The  re- 
flecting power  of  the  mineral  was  compared  with  that  of  a  silver  mirror. 
The  surfaces  were  ground  plane,  but  were  not  always  of  the  highest  polish. 
Since  primarily  we  are  only  concerned  with  the  accurate  location  of  bands 
of  selective  reflection,  the  question  of  polish  is  of  secondary  importance. 

The  spectrometer  slits  were  0.3  mm.,  or  about  2'  of  arc,  as  in  the 
preceding  work.  The  radiation  from  a  Nernst  "heater"  was  projected 
upon  the  reflecting  surfaces  by  means  of  a  mirror  having  a  focal  length  of 
15  cm.  and  an  aperture  of  12  cm. 

The  reflection  spectrum  was  explored  by  means  of  a  Rubens  thermo- 
pile. Although  its  sensitiveness  was  greater  than  the  radiometer  used 
previously,  the  unsteadiness  at  times,  due  to  magnetic  disturbances,  neces- 
sitated repeating  the  observations  at  each  spectrometer  setting.  When 
using  the  radiometer,  it  was  rarely  necessary  to  repeat  the  observations  in 
exploring  the  spectrum  up  to  9  or  10  JJL,  while  in  the  present  work,  using 
a  thermopile,  the  region  beyond  9  p  was  examined  under  the  greatest 
difficulties,  and  in  the  case  of  the  carbonates  no  attempt  was  made  to 
locate  the  band  at  11.4^.  When  the  reflection  faces  were  less  than  the 
standard  size,  2  by  3  cm.,  which  was  frequently  the  case,  the  silver  com- 
parison mirror  and  the  specimen  were  covered  with  diaphragms  having 
equal  openings,  so  that  equal  areas  of  the  two  surfaces  were  exposed. 

9 


CHAPTER  II. 


INFRA-RED   REFLECTION   SPECTRA   OF  VARIOUS   SUBSTANCES. 

CARBONATES. 

SMITHSONITE  (ZnCO3). 
(Stalactitic  crystalline  mass.     From  New  Jersey.     Curve  a,  fig.  i.) 


50% 


40 


030 


,20 


10 


JJ 


The  specimen  examined  was  highly  polished.     The  reflection  curve  is 

strong,  and  is  a  complex  of  two 
bands,  as  will  be  noticed  in 
nearly  all  the  carbonates  exam- 
ined. The  maxima  occur  at 
6.65  and  7.05  fi. 

There  seems  to  be  a  silicate 
calamine  (H2O-2ZnO-SiO2)  hav- 
ing the  same  name.  In  fact 
the  sample  was  purchased  as  a 
silicate.  The  present  examina- 
tion shows  that  the  specimen  ex- 
amined is  the  carbonate  known 
by  that  name.  In  other  words,  this  is  an  independent  method  of  analyzing 
such  a  mineral. 

CERUSSITE  (PbCO3). 
(From  New  South  Wales.     Curve  &,  figs,  i  and  2.) 

This  is  a  rare  mineral  and  the  present  specimen  was  a  fragment  having 
a  surface  about  1.5  by  2  cm. 
The  surface  itself  was  cor- 
rugated, with  several  plain 
highly  polished  plates  about 
2  by  15  mm. 
only    specimen 


45678 
FIG.  i.  —  Smithsonite  (a);  Cerussite. 


25% 


40% 


20 


10 


It  was  the 
obtainable 

and  no  risk  was  taken  in 
attempting  to  grind  it.  The 
reflection  curve  b,  fig.  i, 
therefore  does  not  indicate 
the  true  reflecting  power. 
The  individual  bands,  how- 
ever, are  well  resolved,  the 
maxima  being  at  6.9  and 
7.25  {Ji.  In  fig.  2,  curve  c,  the  reflection  curve  is  drawn  on  a  larger  scale, 
which  emphasizes  these  maxima. 

10 


3  4  5  6  7  8  9  '0/JL 

FIG.  a.  —  Strontianite  (a);  Witherite  (6);  Cerussite. 


CARBONATES. 


II 


STRONTIANITE  (SrCOs). 
(Massive  specimen.     From  Hamm,  Westphalia,  Germany.     Curve  a,  fig.  2.) 

The  specimen  examined  had  a  large,  well-polished  reflecting  surface. 
As  in  all  the  carbonates  studied  the  height  of  the  maximum  reflection  is 
only  about  30  per  cent.  This  is  the  only  carbonate  examined  which  has 
but  one  reflection  maximum,  which  is  at  6.74  //.  It  will  be  noticed  pres- 
ently that  these  maxima  shift  toward  the  long  wave-lengths  with  increase 
in  molecular  weight.  Since  the  intensity  of  the  two  bands  is  unequal,  it  is 
possible  that  this  inequality,  combined  with  the  shift  of  the  maxima,  is  the 
cause  of  the  lack  of  resolution. 

WITHERITE  (BaCO3). 
(Massive,  crystalline.     Curve  &,  fig.  2.) 

The  reflection  decreases  normally  from  4  to  6  p  followed  by  strong 
complex  reflection  band,  with  maxima  at  6.78  and  6.98  /*.  The  latter  band 
is  the  more  intense,  which  is  just  the  opposite  of  the  carbonates,  having  a 
metal  of  less  molecular  weight.  The  specimen  had  an  unusually  high 
polish,  which  accounts,  in  part,  for  the  high  reflection  maximum. 

MALACHITE  (CuO-CO2-H2O). 
(Concretionary  specimen.     From  Burra  Burra,  South  Australia.     Curve  a,  fig.  3.) 

The  specimen  of  malachite  was  highly  polished.  The  reflection  bands 
are  well  resolved,  while  the  second  maximum  is  of  the  usual  intensity  for 
carbonates.  Beyond  10  //  there  appears  to  be  another  band,  but  the  un- 


20% 


3*5  6?  8  9  IOJU 

FIG.  3.  —  Malachite  (a);  Azurite. 

steadiness  of  the  galvanometer  prevented  an  accurate  determination  of 
this  question.  The  maxima  of  malachite  occur  at  6.66  and  7.3  /*,  with  a 
possible  third  band  at  7.8/1. 

AZURITE  (3CuO-2CO2H2O):  CHRYSOCOLLA  (CuSiO3+H2O). 
(From  Lyon  County,  Nevada.     Curve  b,  fig.  3.) 

The  specimen  examined  was  a  mixture  of  azurite  and  chrysocolla,  the 
first  mineral  being  present  in  only  a  small  amount.     No  silicate  bands  at 


12 


INFRA-RED   REFLECTION   SPECTRA. 


8.5  and  9/£  could  be  detected.  The  reflecting  power  is  low,  which  is  no 
doubt  due  to  the  presence  of  the  silicate.  The  maxima  at  6.65,  7.3,  and 
7.8  fj.  are  in  common  with  those  of  malachite.  The  transmission  curve  of 
azurite  is  given  in  the  preceding  volume.  It  shows  the  OH  band  at  3  /x 
and  the  carbonate  bands  at  3.5  and  4  /£. 

DOLOMITE  [CaMg(CO3)2]. 
(A  plane  cleavage  piece.     From  Traversella,  ItaTy.     Curve  a,  fig.  4.) 

Dolomite  is  of  interest  because  it  is  a  double  carbonate  of  Mg  and  Ca. 
The  reflecting  power  is  high.     The  first  band  is  in  common  with  that  of 


567 

FIG.  4.  —  Dolomite  (a);  Siderite. 


CaCO3,  while  the  second  band  is  to  be  found  in  MgCO3.     The  maxima 
occur  at  6.58  and  6.95  /JL. 

SIDERITE  (FeCO3). 
(Cleavage  piece.     From  Allevard,  France.     Curve  b,  fig.  4.) 

The  reflection  curve  of  siderite  is  composed  of  a  complex  maxima 
similar  to  that  of  dolomite.     The  maxima  occur  at  6.6  and  7.1  /<. 


678 
FIG.  5.  —  Calcite  (a);  Magnesite. 

CALCITE  (CaCO3). 
(Curve  a,  fig.  5.) 

The  reflection  curve  is  obtained  from  a  natural  cleavage  face  of  Iceland 
spar.  The  reflection  band  is  complex  with  maxima  at  6.6  and  6.85  /*. 
The  reflection  bands  in  the  deep  infra-red  will  be  noticed  presently.  It  is 


SULPHIDES.  13 

desirable  to  examine  aragonite,  CaCO3,  which  differs  from  calcite  in  its 
crystalline  form,  to  determine  the  effect  of  structure;  this  is  given  in 
Chapter  III.  It  was  shown  in  Carnegie  Publication  No.  35  that  isomeric 
compounds  have  different  transmission  spectra;  and  one  would  expect  also 
the  reflection  spectra  of  isomers  to  be  different.  The  unsteadiness  of  the 
galvanometer  prevented  the  location  of  the  band,  found  by  Aschkinass, 
at  11.4/1.  In  this  region  the  radiometer  would  have  been  more  satisfac- 
tory. 

MAGNESITE  (MgCO3). 
(Massive.     Curve  6,  fig.  5.) 

The  sample  examined  was  an  opaque  white  mass  which  took  a  high 
polish.  The  band  of  selective  reflection  is  very  similar  to  that  of  calcite 
and  consists  of  two  maxima  at  6.5  and  6.8  pt  respectively. 

As  a  whole  the  carbonates  are  conspicuous  for  a  double  band  of  metallic 
reflection  at  6.5  to  7  fi  which  previously  had  not  been  resolved  in  calcite. 
The  shift  of  these  bands  with  increase  in  molecular  weight  of  the  metallic 
ion  will  be  discussed  on  a  later  page.  That  the  shift  is  not  due  to  a  change 
in  the  adjustment  of  the  apparatus  was  verified  at  the  completion  of  the 
observations  by  examining  several  of  the  specimens  in  succession,  when  it 
was  found  that  the  maxima  coincided  with  the  values  first  observed. 

SULPHIDES. 

The  reflection  spectra  of  sulphur  and  of  the  sulphides  of  Zn,  Sb,  Fe, 
and  Pb  were  described  in  the  preceding  volume.  Sulphur  and  sphalerite 
(ZnS)  have  a  low  reflecting  power  of  only  about  8  per  cent  throughout  the 
spectrum  to  15  IJL.  The  remaining  minerals  have  a  high  reflecting  power 
of  32  to  36  per  cent  throughout  the  spectrum  to  15  p,  where  the  reflection 
of  stibnite,  Sb2S3,  seemed  to  decrease.  The  present  examination  includes 
four  new  sulphides. 

MOLYBDENITE  (MoS). 
(Massive  foliated.     From  South  Australia.     Curve  a,  fig.  6.) 

This  mineral  is  very  soft,  like  graphite.  The  specimens  were  folded 
and  distorted  so  that  it  was  not  possible  to  grind  a  surface  parallel  to  a 
cleavage  plane.  The  surface  took  a  high  polish,  but  no  luster  as  is  found 
in  the  cleavage  lamina.  The  reflection  curve  is  fairly  uniform  beyond  4  /*, 
which  would  indicate  that  the  lack  of  polish  had  no  serious  effect  beyond 
this  point.  The  reflecting  power  is  low  and  uniform  (18  to  20  per  cent, 
as  compared  with  stibnite,  35  per  cent)  throughout  the  spectrum  to  14  p, 
which  is  to  be  expected  from  its  electrical  conductivity.  (For  further 
references  to  electrical  conductivity,  etc.,  see  Carnegie  Publication  No.  65, 
p.  93;  also  Konigsberger,  Jahrb.  Radioaktivitat  &  Elektronik,  vol.  4, 
p.  161,  1907.) 


INFRA-RED   REFLECTION   SPECTRA. 


PYRRHOTITE  [(Fe,  Ni)S]. 
(Massive.     From  Sudbury,  Ontario.     Curve  6,  fig.  6.) 

The  specimen  examined  did  not  appear  so  highly  polished  as  molyb- 
denite. It  is  composed  of  a  bright  metal  and  a  darker  background.  The 
reflecting  power  is  higher  than  that  of  MoS,  and  increases  rapidly  with 
wave-length  through  the  spectrum  of  14  /*. 

CHALCOCITE  (Cu2S). 
(Curve  c,  fig.  6.) 

The  reflecting  power  rises  rapidly  from  15  per  cent  at  i  fj.  to  45  per 
cent  at  4  /z,  while  beyond  7  /*  the  reflecting  power  has  a  fairly  constant 

60%r— 


a 


0  /  2          3          4          5          6  7          S          3          IO         I!          12         13        14-JJ. 

FIG.  6.  —  Molybdenite  (o);  Pyrrhotite  (6);  Chalcocite  (c);  Covellite. 

value  of  55  per  cent.     This  substance  is  black,  not  unlike  magnetite  and 
the  Siberian  graphite  previously  described.     The  polish  was  higher  than 

in  pyrrhotite. 

COVELLITE  (CuS). 

(From  Anaconda  Mine,  Butte,  Montana.     Curve  d,  fig.  6.) 

This  mineral  is  of  a  steel-blue  color,  and  appears  as  compact  as  steel. 
The  reflecting  power  rises  rapidly  at  i  and  2  p.  and  then  more  slowly  to 
14  /*,  where  it  amounts  to  75  per  cent. 

As  a  whole,  the  examination  of  the  sulphides  shows  that  the  sulphur 
atom  has  merely  reduced  the  reflecting  power  of  the  metal,  but  has  not 
brought  about  any  bands  of  selective  reflection  in  the  region  examined. 
In  the  case  of  sphalerite  (ZnS)  the  sulphur  atom  has  introduced  into  the 
metal  a  property  which  is  to  be  found  only  in  non-metals,  viz,  low  re- 
flecting power  and  selective  absorption  in  the  infra-red.  Of  the  elements 
which  are  on  the  border  line  between  metals  and  non-metals,  selenium 
behaves  like  a  metal  in  its  high  reflecting  power  (see  fig.  7)  and  absence 
of  infra-red  absorption  bands,  while  iodine  and  sulphur  (non-metals)  have 
absorption  bands  in  this  region. 


OXIDES.  15 

SELENIUM  (Se). 
(Curve  d,  fig.  7.)  * 

Selenium  has  practically  the  same  reflecting  power  (18  to  20  per  cent) 
as  molybdenite.  Pfund  (Johns  Hopkins  Univ.  Circular  No.  4,  1907) 
has  made  a  thorough  investigation  of  the  infra-red  polarization  of  this 
substance. 

OXIDES. 

In  the  previous  examination  only  quartz  (SiO2)  represented  this  im- 
portant group  of  minerals. 

MAGNETITE  (Fe3O4). 
(From  Port  Henry,  Essex  County   New  York.     Curve  a,  fig.  7.) 

The  surface  examined  was  a  triangular  crystal,  face  about  3  cm.  on 
an  edge.  The  specimen  was  very  homogeneous,  but  did  not  take  a  high 
polish,  sufficient  to  observe  the  reflected  image  of  an  object  at  a  small 
angle  of  incidence.  The  reflecting  power  rises  uniformly  throughout  the 
spectrum,  showing  that  the  observations  are  affected  by  the  lack  of  polish. 

HEMATITE  (Fe2O3). 
(From  Cumberland,  England.     Curve  6,  fig.  7.) 

The  specimen  examined  was  a  very  dense  homogeneous  concretion, 
polished  parallel  to  the  radius  of  growth.  The  surface  had  a  high  polish 
(better  than  that  of  magnetite),  reflecting  a  strong  image  of  a  source  at  a 


40% 


01  2x34-56789/0/7/2 

FIG.  7.  —  Magnetite  (a);  Hematite  (6);  Chromite  (c)\  Selenium. 

small  angle  of  incidence.  The  reflecting  power,  in  the  infra-red,  is  far 
below  that  of  magnetite,  although  it  is  higher  in  the  visible.  The  reflecting 
power  is  uniformly  12  per  cent,  from  which  it  would  appear  that  this  is 
the  normal  value  for  hematite. 

CHROMITE  (FeO,  CraO3). 
(From  Lancaster  County,  Pennsylvania.     Curve  c,  fig.  7.) 

The  surface  of  this  dark  specimen  was  mottled,  parts  of  which  took  a 
high  polish.  The  reflecting  power  is  uniformly  4  per  cent  throughout  the 
spectrum. 

It  is  of  interest  to  note  that  the  iron  oxides  show  no  bands  of  selective 
reflection  in  the  region  of  the  spectrum  up  to  15  ;*;  zincite,  ZnO,  fig.  9,  is 
another  example. 


i6 


INFRA-RED   REFLECTION   SPECTRA. 


SCHEELITE  (CaWO4). 

(Massive.     From  Armidale,  New  South  Wales.     Curve  a,  fig.  8.) 

The  reflection  power  of  this  mineral  behaves  in  the  usual  manner, 
being  low  in  the  region  of  the  spectrum  up  to  n  /*,  followed  by  a  strong 
complex  band  of  selective  reflection,  the  maximum  of  which  extends  from 
11.3  to  12.5  p,  the  possible  single  maxima  being^at  11.3,  u.8,  and  12.4 ,«. 
The  reflecting  power  is  unusually  low  on  the  long  wave-length  side  of  the 
reflection  band. 

ZIRCON  (ZrSiO4). 
(Near  Eganville,  Renfrew  County,  Ontario.     Curve  6,  fig.  8.) 

The  specimen  examined  was  a  large  rectangular  cleavage  piece,  about 
3  by  3  cm.,  of  brownish  color,  semitransparent  in  thin  sections. 

Although  this  is  a  double  oxide  of  zircon  and  silicon,  the  reflection  curve 
is  entirely  different  from  the  silicates  previously  studied.  The  strong  band 
of  selective  reflection  occurs  farther  toward  the  long  wave-lengths,  the 
single  maxima  (not  well  resolved)  being  at  10.1,  10.6,  and  n  with  a  pos- 


6O% 


40 

I 


20 


10 


3  4  5  6  7  8  9  IO  II  12  13          14/Z 

FIG.  8.  —  Scheelite  (a);  Zircon. 

sible  band  at  11.7  /x.  As  a  whole  the  reflection  curve  is  exactly  the  same 
as  that  of  willemite,  Zn2SiO4,  previously  studied.  In  the  latter,  the  bands 
are  well  resolved  and  occur  at  10.1,  10.6,  n.o,  and  n.6  ft.  From  this  it 
would  appear  that  in  these  two  minerals  the  effect  of  SiO2  is  different 
from  that  found  in  quartz  and  in  the  minerals  commonly  known  as  sili- 
cates, viz,  silicates  of  Ca,  Mg,  Fe,  etc.  Some  of  the  latter,  however,  have 
the  last  band  lying  close  to  the  first  band  in  the  present  silicates. 

WULFENITE  (PbMoO4). 

(Red  Cloud  Mine,  Yuma  County,  Arizona.     Curve  a,  fig.  9.) 

This  is  a  chrome-red  crystal.  A  natural  crystal  face,  1.5  by  2  cm., 
having  a  high  polish,  but  not  perfectly  plane,  was  examined.  In  spite  of 
this  the  reflection  curve  is  one  of  the  most  remarkable  yet  found.  The 
band  of  metallic  reflection  is  almost  as  strong  as  quartz.  There  are  two 


OXIDES.  17 

maxima,  at  11.75  an^  13-1  /*,  respectively.  The  reflecting  power  decreases 
uniformly  from  12  per  cent  at  4  //  to  10  per  cent^at  8  /£,  passes  through  a 
minimum  of  4  per  cent  at  10.8  /JL,  then  suddenly  rises  to  78  per  cent  at 
11.75  AC 

RUTILE  (TiO2). 
(Graves  Mountain,  Lincoln  County,  Georgia.     Curve  6,  fig.  9.) 

The  surface  of  the  specimen  examined  was  a  natural  crystal  face,  area 
about  1.4  by  1.8  cm.,having*a  high  polish,  but  uneven  and  containing  cracks. 

The  reflection  curve  is  the  most  unusual  of  all  examined  in  that  the 
band  of  selective  reflection  occurs  almost  at  the  working  limit  of  a  rock- 
salt  prism.  The  maximum  is  broad  and  is  fairly  well  defined  at  13.6  /*. 


80% 


4-  5  6  7  8  9  10          II 

FIG.  9.  —  Wulfenite  (a);  Rutile  (6);  Zincite. 


/5/z 


ZINCITE  (ZnO). 
(From  Franklin,  New  Jersey.     Curve  c,  fig.  9.) 

The  specimen  examined  was  massive,  having  a  red  color  and  a  dull 
polish.  The  surface  was  full  of  cracks,  which  would  materially  reduce 
the  reflecting  power.  The  reflecting  power  is  low  and  uniform  throughout 
the  region  of  the  spectrum  examined.  No  bands  of  selective  reflection 
were  found.  In  this  respect  zincite  is  similar  to  iron  oxide,  but,  on  account 
of  its  low  reflecting  power,  it  is  to  be  classed  with  the  "  insulators,"  and 
hence  one  would  expect  to  find  bands  of  selective  reflection  in  the  infra-red. 

CORUNDUM  (A12O3). 
(Craigmont,  Renfrew  County,  Ontario.     Curve  6,  fig.  10.) 

This  specimen  was  an  opaque  crystal,  of  which  a  cleavage  surface, 
about  2  by  3  cm.  in  area,  was  examined.  The  surface  contained  striae, 
but  otherwise  had  a  high  polish.  The  reflecting  power  is  unusually  low, 
except  in  the  region  of  selective  reflection,  when  it  is  quite  high.  The 
region  of  selective  reflection  is  wide  with  maxima  at  n.o,  n.8,  and  13.5  JJL. 


i8 


INFRA-RED  REFLECTION  SPECTRA. 


ALUMINUM  SILICATES. 

There  is  no  special  reason  for  thus  classifying  the  following  minerals, 
except  that  most  of  them  are  quite  unlike  the  commoner  silicates  previously 

examined. 

CYANITE  (Al2O3SiO2). 

(Yancy  County,  North  Carolina.     Curve  a,  fig.  10.) 

The  color  of  this  flat  crystal  was  light  green.0  The  flat  crystal  face, 
2.5  by  3.5  cm.,  was  ground  but  did  not  have  a  high  polish.  The  band  of 
selective  reflection  has  three  sharp  maxima,  at  9.3,  9.78,  and  10.28;*, 


60% 


67  8  9          10          II 

FIG.  10.  —  Cyanite  (a);  Corundum. 


15/1 


respectively,  and  is  not  unlike  that  of  willemite  (Zn2SiO4),  except  that  in 
the  latter  the  whole  band  is  shifted  to  longer  wave-lengths  with  the  maxima 
at  10.1,  10.6,  and  u  /*. 

BERYL  [BeaAl^SiOa^]. 
(From  North  Carolina.     Fig.  u.) 

The  surface  examined  was  ground  on  a  crystal  face.  The  specimen 
was  opaque  and  green  in  color.  The  reflection  curve  follows  the  usual 
course  with  a  band  of  selective  reflection  extending  from  8  to  10.5  /*.  The 


A 

( 

A 

i 

J 

\ 

I 

\ 

V 

J 

3             4-             5             6             7             8            9             IO            II            12          13 

FIG.  ii.  —  Beryl. 


maxima  are  not  so  high  as  usual  with  silicates,  but  they  are  sharp  and 
occur  at  8.15,  9.2,  9.9,  and  10.4  /*. 


SILICATES. 


TOPAZ  [(Al, 
(Villa  Rica  Mines,  Geraes,  Brazil.     Curves  a  and  b, 


J.    12.) 


The  specimen  from  Brazil  was  a  yellow  rectangular  prism  with  faces 
i  by  2.5  cm.  The  natural  face,  which  had  a  high  polish,  was  examined. 
The  reflection  curve  a,  fig.  n,  is  low  throughout  the  spectrum,  except  in 


80% 


70 


the  region  of  selective  reflection,  which  extends  from  10  to  11.5^  with 
unresolved  maxima  at  10.05,  10.6,  10.9,  and  11.3/4. 

The  white  topaz  examined,  curve  b,  fig.  u  (in  its  highest  part),  is  an 
almost  exact  reproduction  of  the  quartz  curve,  with  the  exception  that  the 
latter  is  shifted  to  the  longer  wave-lengths.  The  white  topaz,  which  was 
ground  and  had  a  fairly  high  polish,  has  sharp  maxima  at  9.9,  10.45,  an(^ 
ii  H  with  a  possible  band  at  12  /*.  The  band  at  u  /*  is  in  common  with 
that  of  several  other  silicates. 

SODIUM  SILICATE  (Na2SiO3). 
(Curve  b,  fig.  13.) 

The  curve  of  liquid  glass  is  similar  to  that  of  the  glasses  previously 
examined.  This  is  one  of  the  simplest  obtainable  chemical  compounds  of 
the  silicates,  and  is  further  evidence  that  the  silicon  oxide  radical  is  not  so 
constant  in  its  behavior  toward  heat-waves  as  was  found  in  the  CO2  radical 

of  the  carbonates. 

SPODUMENE  [LiAl(SiO3)2]. 

(From  Pennington  County,  South  Dakota.     Curve  a,  fig.  13.) 

This  mineral  is  of  the  same  composition  as  kuntzite,  the  transmission 
curve  of  which  is  given  on  a  later  page.  The  specimen  examined  was  a 
polished  cleavage  piece  from  a  large  gray  crystal.  The  maxima  of  the 
selective  reflection  bands  occur  at  9.18,  9.7,  and  10.4  /<,  respectively. 


20 


INFRA-RED   REFLECTION   SPECTRA. 


In  conclusion,  it  may  be  said  that,  from  all  of  the  silicates  examined, 
there  is  no  regularity  in  the  position  of  the  maxima  such  as  obtains  in  the 
sulphates  and  the  carbonates.  It  is  true  that  in  a  few  cases  the  maxima 
are  in  common,  but,  on  the  whole,  the  evidence  indicates  that  there  is  no 
uniform  group  of  atoms  acting. in  common  as  is  found  in  the  sulphates 
and  carbonates.  In  other  words,  the  silicon  radical  is  different  in  the 


OU/o 

40 

C 
0 
1£30 

*> 

ts 

a:  20 

10 
i 

A/ 

\ 

'/ 

A 

k* 

I 

A 

l\ 

^ 

- 

a 

r3^\ 

^ 

\ 

>- 

\            4-           S            f>            7            8            9            IO           II           li 

H- 

FIG.  13.  —  Spodumene  (a);  Sodium  silicate. 


different  minerals.  This  seems  to  be  the  necessary  interpretation  to  be 
given,  for  it  has  been  impossible  to  work  out  a  relation  with  the  molecular 
weight  of  the  molecule,  such  as  exists  in  the  carbonates  and  sulphates,  to 
be  discussed  on  a  later  page. 


CHAPTER  III. 

MINUTE  EXAMINATION  OF  THE  REFLECTION  BANDS  OF  QUARTZ 
AND  OF  THE  CARBONATES. 

Having  assembled  a  fluorite  prism  and  bolometer  for  radiation  work, 
it  seemed  worth  while  to  spend  some  time  on  the  examination  of  the  reflec- 
tion bands  at  6  to  7  /JL  in  the  carbonates,  and  at  8.5  to  9  /<  in  quartz.  The 
fluorite  prism  had  a  circular  aperture  of  3.3  cm.,  angle  60°,  and  was  per- 
fectly clear.  It  was  mounted  on  the  spectrometer  used  in  the  previous 
work  and,  for  the  regions  of  the  spectrum  examined,  the  dispersion  was 
from  4  to  6  times  that  of  rock  salt.  On  account  of  the  large  dispersion 
and  the  small  prism  face,  the  deflections  were  only  about  one-tenth  that 
previously  used.  The  glower  of  the  Nernst  lamp  was  used  in  order  to 
obtain  a  sufficiently  strong  source  of  radiation.  The  main  objection  in 
using  a  glower  is  its  narrowness,  which  requires  greater  care  in  maintaining 
a  constant  adjustment.  The  bolometer  strip  was  about  0.5  mm.  or  4'  of 
arc,  while  the  temperature  sensibility  was  i  mm.  =  5°  X  io~5C.  A  radi- 
ometer would  have  been  more  satisfactory,  but  the  bolometer  was  con- 
veniently at  hand.  With  this  greater  dispersion  it  was  found  that  the 
reflection  bands  of  some  of  the  carbonates  are  quite  complex,  while  in 
others  there  is  but  one  band. 

QUARTZ.  1 

(Crystal  cut  perpendicular  to  optic  axis.     Curve  b,  perfectly  clear  specimen  of 
quartz  glass,  fig.  14.) 

It  has  repeatedly  been  noticed  that  the  various  silicates  have  quite 
different  reflection  spectra,  while  in  the  carbonates  and  in  the  sulphates 
the  spectra  show  great  similarity.  It  was  therefore  assumed  that  the 
silicon  oxide  radical  is  differently  united  in  the  different  silicates.  All  the 
evidence  obtainable,  without  exception,  of  substances  in  the  solid  or  liquid 
(crystalline  or  amorphous)  condition,  shows  that  crystallographic  form 
does  not  explain  these  anomalies;  neither  will  the  slight  impurities  present 
in  many  of  the  silicates  explain  them. 

Pfund  2  found  identical  reflection  bands  of  sodium-potassium  tartrate 
(C8H4K  NaO6  +  4H2O)  in  the  form  of  a  polished  crystal,  and  also  in  the 
molten  condition.  His  reflection  maxima  of  molten  nitroso-dimethylani- 

1  The  writer  is  indebted  to  Dr.  Day  and  Mr.  Sheppard,  of  the  Geophysical  Laboratory  of 
the  Carnegie  Institution  of  Washington,  for  the  sample  of  quartz  glass. 

2  Pfund,  Astrophys.  Jour.,  24,  p.  31,  1906. 


22 


INFRA-RED  REFLECTION  SPECTRA. 


80% 


3456789 
FIG.  14.  —  Quartz:  (a)  Crystalline;  (b)  Amorphous. 


line  [(CH3)2NC6H4NO]  coincide  with  the  absorption  maxima  found  by 
the  writer  for  a  solid  film  of  this  compound.  Furthermore,  the  writer 
found  that  reflection  spectra  of  solids  in  solution  may  or  may  not  be  iden- 
tical with  that  of  the  solid,  note- 
worthy examples  being  the 
sulphates  of  copper  and  of  so- 
dium. The  cause  of  the  reflec- 
tion baflds  is  therefore  to  be 
sought  within  the  molecule. 

From  the  fact  that  the  phys- 
ical and  chemical  properties  of 
quartz-glass  are  different  from 
the  crystal,  one  would  infer 
that  there  is  a  difference  in  the 
molecular  structure.  This  is 
well  illustrated  in  fig.  14.  The 
reflection  spectrum  of  the  amor- 
phous material  is  entirely  differ- 
ent from  that  of  the  crystal.  It 
seems  to  have  no  connection  with 
any  of  the  silicates  examined.  The  maxima  of  the  reflection  spectrum  of 
crystalline  quartz  occur  at  8.4  and  9.02  /.i,  while  those  of  the  amorphous 
quartz  are  found  at  7.8,  8.4,  and  8.8  /£,  respectively.  The  intensity  of  the 
maxima  of  bands  of  the  amorphous  quartz  is  not  so  great,  neither  does  the 
reflecting  power,  in  the  region  of  the  spectrum  just  preceding  a  reflection 
band,  fall  to  so  low  a  value  as  in  crystalline  quartz. 

In  this  connection  it  may  be  noticed  that  the  various  kinds  of  glass 
(sodium  silicates)  previously  examined  have  similar  reflection  spectra. 

CALCITE  AND  ARAGONITE  (CaCO3). 
(Fig.  15.     Curve  a,  calcite;  curve  6,  aragonite.) 

From  the  fact  that,  although  the  carbonates  examined  belong  to  only 
two  crystal  systems,1  the  reflection  spectra  are  different,  one  would  infer 
that  the  cause  is  not  to  be  attributed  to  crystalline  form.  Previous  exami- 
nations have  shown  that  isomeric  substances  have  different  spectra.  One 
would  therefore  expect  to  find  the  spectra  of  calcite  and  aragonite  to  be 
different,  as  was  previously  found  for  orthoclase  and  microclene  (KAlSi3O8) 
at  8  to  10  //.  The  fact  that  the  substances  are  inorganic,  and  that  the  re- 
flection bands  are  far  in  the  infra-red,  is  of  interest  but  non-essential. 

In  fig.  15,  curve  a  gives  the  reflection  from  the  plane,  highly  polished 

1  To  the  rhombohedral  system  belong  CaCO3  (calcite),  MgCO3,  FeCO3,  and  ZnCO3;  to 
the  orthorhombic  system  belong  CaCO3  (aragonite),  BaCO3,  SrCO3,  and  PbCO3.  In  the 
former  group  the  band  toward  the  shorter  wave-length  appears  to  be  the  most  intense,  while 
in  the  latter  group  the  bands  seem  sharper  and  of  more  uniform  intensity. 


CARBONATES. 


cleavage  face  of  Iceland  spar.  With  the  larger  dispersion  the  band  pre- 
viously found  at  6.6  /*  is  now  resolved  into  two  bands  with  maxima  at 
6.5  and  6.6  /*,  respectively.  The 
region  at  7  ^  is  evidently  still 
more  complex,  but  not  resolved 
even  with  this  greater  dispersion. 
In  curve  b  is  given  the  reflection 
spectrum  of  a  clear  crystal  of 
aragonite.  The  crystal  face  was 
small,  only  about  6  by  15  mm., 
and  since  the  area  of  the  com- 
parison mirror  was  not  reduced 
in  like  proportion,  the  reflecting 
power  of  the  maxima  is  really 
higher  than  here  given.  In  fact, 
for  a  well-polished  crystal  the 


60 


50 


30 


20 


10 


.2  .4  .6  .8  7.0 

FIG.  15.  —  Calcite  (a);  Aragonite. 


7.4-JJ. 


reflecting  power  is  no  doubt  as 
high  as  for  calcite.     The  impor- 
tant point  of  interest  in  the  pres- 
ent examination  is  that  aragonite  has  only  two  reflection  maxima,  of  about 
equal  intensity,  and  located  at  6.53  and  6.75  /*,  respectively. 

MAGNESITE  (MgCO3). 
(Curve  a,  fig.  16.) 

This  sample  was  previously  examined  (fig.  5).     In  the  present  curve 
there  are  three  bands  with  maxima  at  6.42,  6.65,  and  6.9  /*. 

•60%. 


50 


30 


20 


\ 


6          0.2          .4  .6  .3  7  .Z         7.4JU 

FIG.  1 6.  —  Magnesite  (a);  Smithsonite. 

SMITHSONITE  (ZnCO3). 
(Curve  b,  fig.  16.) 

This  sample  was  previously  examined  (fig.  i),  when  two  maxima  were 
found.  In  the  present  examination  the  bands  are  more  resolved,  but  the 
maxima  occur  at  6.65  and  7.05  /*,  respectively,  as  in  the  previous  investigation. 


24  INFRA-RED   REFLECTION   SPECTRA. 

STRONTIANITE  (SrCO3). 
(Curve  o,  fig.  17.) 

The  band  at  6.68  /*  is  not  resolved  even  with  the  large  dispersion  of 
fluorite.  The  band  is  symmetrical,  as  previously  found  in  fig.  2. 

WITHERITE  (BaCO3). 
(Curve  6,  fig.  17.) 

The  pair  of  bands  is  somewhat  better  resolved,  with  maxima  at  6.7 
and  6.97  JJLJ  respectively,  the  same  as  was  found  in  the  previous  examination 
of  this  same  specimen  (see  fig.  2).  The  reflecting  power  is  slightly  lower 


c30 
o 

X  —  \ 

/ 

\ 

b 

\^ 

«*- 
Q> 

<r 

10 

// 

'f 

5 

k 

/ 

7 

V 

.2  .4  -6  .8  7  .2 

FIG.  17.  —  Strontianite  (a);  Witherite. 


7.4/4 


but  that  is  to  be  attributed  to  the  difference  in  the  adjustment,  i.e.,  the  faces 
of  the  comparison  mirror  and  of  the  substance  may  not  have  been  in  the 
same  plane.  As  a  result  they  would  not  reflect  the  same  part  of  the  image 
of  the  glower  upon  the  spectrometer  slit.  This  is  of  minor  importance 
since  we  are  not  concerned  with  the  absolute  reflecting  power. 

On  a  subsequent  page  it  is  shown  that  an  emission  band  of  Adularia 
occurring  at  2.9  /*  is  shifted  to  3.2  /*  in  the  absorption  spectrum  of  the  crys- 
talline material.  Using  polarized  energy,  the  reflection  bands  of  the  car- 
bonates depend  upon  the  direction  of  vibration  of  the  incident  energy. 
The  author  therefore  hopes  to  investigate  water  solutions  to  learn  the  be- 
havior of  these  reflection  bands,  using  polarized  radiation. 


V 


CHAPTER  IV. 

REFLECTION    SPECTRA    IN   THE    EXTREME    INFRA-RED. 

From  the  foregoing  work  on  selective  reflection  in  which  a  single  re- 
flecting surface  was  used,  it  is  apparent  that  by  successive  reflection  of 
heat-waves  from  several  surfaces  there  will  remain  only  the  residual  rays 
lying  in  the  region  of  selective  reflection.  It  was  noticed  that  the  reflecting 
power  in  the  region  of  4  to  8  JJL  is  only  about  -j1^  that  of  the  band  of  selec- 
tive reflection.  Hence,  after  reflecting  from  three  surfaces  the  intensity 
would  be  only  To10"o>  anc^  a^ter  ^ve  reflections  only  T-o  o^Fo- 

Rubens  and  Nichols  1  were  the  first  to  apply  this  method  in  locating 
the  maxima  of  the  residual  rays  of  a  series  of  substances  including  quartz, 
mica,  fluorite,  rock-salt,  sylvite,  crown  and  flint  glass,  sulphur,  alum,  and 
calcite.  By  using  a  grating  of  fine  wire  they  were  able  to  extend  their 
observations  to  61  //,  the  longest  heat-waves  yet  identified.  Of  the  above- 
mentioned  substances,  only  the  first  four  were  found  to  have  bands  of 
residual  rays  in  the  extreme  infra-red. 

Aschkinass 2  did  some  further  work  on  this  subject,  examining  marble, 
calcite,  selenite,  alum,  sodium  bromide,  and  potassium  bromide.  He 
found  a  band  of  residual  rays  at  29.4  /*  in  marble,  and  showed  that  similar 
bands  exist  in  the  bromides,  the  maxima  of  which  lie  beyond  60  p. 

From  the  results  obtained  with  the  silicates  (see  Carnegie  Publication 
No.  65,  pp.  80  to  90),  especially  glass,  it  becomes  apparent  that  one  can 
hardly  expect  to  locate  bands  of  residual  rays  in  the  extreme  infra-red,  even 
at  1 8  to  20  /*,  unless  they  are  much  more  intense  than  those  found  in  the 
region  of  8  to  10  //.  For  it  is  necessary  to  have  several  reflecting  surfaces 
to  eliminate  the  large  amount  of  energy  of  short  wave-lengths  as  compared 
with  the  small  amount  to  be  measured,  of  the  long  wave-lengths.  As  is 
well  known  now,  in  the  case  of  quartz,  rock-salt,  and  sylvite,  this  is  an 
easy  matter  on  account  of  the  high  reflecting  power  of  these  bands.  For 
example,  for  the  band  at  61  /*,  which  was  examined  after  reflection  from 
five  surfaces  of  sylvite,  it  was  found  that  the  reflecting  power  of  sylvite  is 
80  per  cent.  The  galvanometer  deflections  were  only  about  5  mm.  in 
the  maximum  of  the  band.  If  the  reflecting  power  were  only  one-half 
this  amount  (cf.  the  silicates),  the  galvanometer  deflection  after  five 
reflections  would  be  one  thirty-second  of  5  mm.,  which  could  not  be 

1  Rubens  and  Nichols,  Ann.  der  Phys.  (3),  60,  p.  418,  1897. 

2  Aschkinass,  Ann.  der  Phys.  (4),  I,  p.  42,  1900. 

25 


26  INFRA-RED   REFLECTION   SPECTRA. 

observed  with  accuracy.  In  the  case  of  the  silicates  (as  compared  with 
quartz),  where  the  height  of  the  reflection  band  at  8  to  lo/z  is  always 
considerably  less  than  50  per  cent,  after  three  reflections  the  galvanometer 
deflections  would  be  only  i  or  2  mm.  at  1 8  to  20  //.  It  does  not  follow, 
therefore,  because  no  residual  rays  of  a  substance  are  to  be  found  in  the 
region  of  18  to  20  p,  as  was  the  case  in  the  present  examination,  that  no 
bands  exist,  but  that  they  are  too  weak  to  be  measured.  From  the  simi- 
larity of  the  spectra,  throughout  the  infra-red,  of  great  groups  of  chemically 
related  compounds,  and  from  the  fact  that  quartz  and  mica  have  bands  of 
residual  rays  in  the  region  of  18  to  20  /*,  it  is  to  be  assumed  that  the  sili- 
cates, in  general,  are  selectively  reflecting  in  this  region.  The  fact  that 
no  bands  were  found  is  to  be  attributed  to  the  weakness  of  the  reflection 
bands. 

As  a  whole,  however,  the  reflecting  power  of  some  of  these  bands  is 
very  high.  One  has  really  no  conception  of  the  state  of  affairs  until  he 
examines  a  substance  like  quartz.  The  phenomenon  is  so  easily  observed 
with  quartz  that  it  might  well  serve  as  a  general  laboratory  experiment. 
In  regard  to  the  ease  of  observing  in  the  infra-red,  the  writer's  experience 
has  extended  from  the  optical  region  into  the  most  remote  infra-red,  and 
it  may  be  said  that  more  difficulty  was  experienced  in  the  region  of  12  to 
15  //,  on  account  of  the  absorption  of  the  rock-salt  prism,  than  in  the 
region  of  greater  wave-lengths.  The  actual  intensity  in  the  emission 
spectrum  differs  greatly  throughout  the  spectrum.  For  example,  Rubens 
and  Aschkinass  (loc.  cit.)  compute  that  for  a  temperature  of  2000°  the 
maximum  emission  at  1.5  JJL  is  800,000  times  as  great  as  at  60  /*. 

APPARATUS  AND  METHODS. 

In  the  present  examination  the  usual  methods  of  procedure  were 
employed.  The  spectrometer  was  the  one  used  in  previous  work.  The 
spectrum  was  produced  by  means  of  a  wire  grating  G,  fig.  18.  The  grating 
was  made  by  the  well-known  method  of  winding  two  copper  wires  of  the 
same  diameter  on  a  brass  frame.  One  wire  was  then  unwound  and  the 
remaining  one  was  fastened  to  the  frame  by  means  of  an  electrolytic  deposit 
of  copper.  The  strands  were  then  cut  from  one  side  of  the  frame.  The 
wire  was  not  of  uniform  thickness,  so  that  the  grating  was  far  from  perfect. 
Such  a  grating  has  the  well-known  property  of  producing  only  the  odd 
order  of  spectra.  On  account  of  its  imperfections  it  was  not  possible  to 
make  accurate  measurements  on  orders  higher  than  the  seventh,  using  a 
Bunsen  sodium  flame.  Two  gratings  were  employed,  the  one  (No.  2) 
having  a  constant  of  ^=0.2120  mm.,  for  the  other  £  =  0.3279  mm., 
determined  with  the  sodium  flame.  The  coarser  grating  was  the  more 
uniform  in  its  individual  windings,  but  in  one  region  the  wires  were  more 
closely  but  very  regularly  wound.  The  magnesite  band  at  29.4 /*  was 
apparently  double,  at  least  very  wide,  just  as  though  this  grating  spectrum 


RESIDUAL   RAYS.  27 

contained  "ghosts."  For  the  main  part  of  the  investigation  grating  No.  2 
was  used,  which  on  account  of  the  finer  wires  was  not  so  regular  in  its 
winding.  Using  three  reflections  from  quartz  it>was  possible  to  observe 
the  third  order  maximum  of  the  band  of  wave-length,  9.05  /*;  in  all  other 
cases,  the  third  order  spectrum  was  too  weak  for  observation.  A  grating 
having  wires  of  more  uniform  diameter  and  more  uniformly  wound,  e.g., 
wound  on  a  screw  thread,  would  no  doubt  have  been  more  efficient  in 
producing  a  pure  spectrum. 


FIG.  18. 

The  spectrometer  arm  carrying  the  Nernst  "heater,"  the  slit  5t  and 
the  mirror,  mlt  was  movable.  The  plane  of  the  grating  was  made  normal 
to  the  beam  of  light  by  placing  a  mirror  on  the  wires  and  revolving  the 
grating  until  an  image  of  the  slit,  Slt  was  projected  back  upon  the  slit. 
It  was  not  convenient  to  have  the  collimating  mirror  and  the  grating  revolve 
about  the  spectrometer  axis,  which  is  necessary  to  maintain  the  same  grat- 
ing constant.  Accordingly  the  grating  was  kept  in  a  fixed  position  and 
the  spectrometer  arm,  carrying  the  mirror  and  source,  was  revolved 
about  it. 

The  apparatus  was  calibrated  by  locating  the  maxima  of  the  selective 
reflection  bands  of  substances  previously  examined  by  Rubens  and  Nichols 
and  by  Aschkinass,  viz,  quartz  at  8.7  and  20.75  fa  mica  at  J^-4  and  21.25  fa 
fluorite  at  24.4  /*,  and  calcite  at  29.4  /JL.  The  calibration  curves  are  shown 
in  fig.  19,  where  curve  a  is  for  the  grating  having  a  constant  ^=0.3279  mm. 
(for  Na),  and  curve  b  is  for  the  grating  (No.  2)  having  a  constant  K= 0.2120 
mm.  (for  Na).  In  this  curve  the  abscissas  are  the  maxima  of  reflection 
bands  and  the  ordinates  are  the  rotations  of  the  spectrometer  arm  from 
the  zero  position.  Blocks  of  wood  with  vertical  metal  plates,  having 
openings  2  by  3  to  4  by  5  cm.  were  mounted  securely  upon  a  board,  as 
shown  at  rlt  rz,  rs,  r4.  The  minerals  were  secured  to  the  back  of  the  metal 


28 


INFRA-RED   REFLECTION    SPECTRA. 


plates  by  means  of  soft  wax,  the  plane  faces  being,  of  course,  placed  over 
the  openings  in  the  plates.  In  case  less  than  four  reflecting  surfaces  were 
available,  aluminum  mirrors  were  placed  in  the  remaining  holders.  A 
short  focus  mirror  projected  an  image  of  the  slit,  S2,  upon  an  improved 
iron-constantan  thermopile  of  20  junctions.  The  wire  used  in  the  ther- 
mopile was  only  0.075  mm-  m  diameter.  This  eliminated  heat  conduction 
and  there  was  no  drift  of  the  zero.  The  galvanometer  suspension  weighed 
about  10  mg.,  which  eliminated  the  effect  of  earth  tremors.  This,  how- 
ever, necessitated  lengthening  the  period  to  gain  great  sensitiveness.  The 


a  12.  /6  ZQ  24  %g  syi, 

I'IG.  19.  —  Wke-grating  calibration. 

thermopile  was  in  a  metal  case,  wrapped  in  felt,  and  the  complete  outfit 
was  perfectly  steady.  It  was,  therefore,  possible  to  increase  the  period  of 
the  galvanometer  to  20  to  30  seconds  (single  swing)  when  its  sensitiveness 
was  t=5Xio~n  ampere  on  a  scale  at  i  m.  The  resistance  of  the  ther- 
mopile was  8.9  ohms.  The  galvanometer  resistance  was  5  ohms,  from 
which  it  was  computed  that  a  deflection  of  i  mm.  =  7Xio~7  °C.  for  a  scale 
at  i  m. 

From  this  it  will  be  seen  that  the  temperature  sensitiveness  of  the 
instrument  was  as  great  as,  and  in  some  instances  greater  than,  in  previous 
investigations.  Hence,  it  appears  that  in  numerous  cases  the  absence  of 
reflection  bands  is  to  be  attributed  to  some  property  in  the  material  rather 
than  to  a  fault  in  the  instruments. 

The  whole  apparatus  was,  of  course,  thoroughly  protected  from  stray 
light  by  providing  numerous  black  pasteboard  screens. 


RESIDUAL   RAYS. 


RESIDUAL  RAYS1  FROM  MICA. 

MUSCOVITE  MICA  [H2KAl(SiO4)3]. 

In  fig.  20  are  plotted  the  spectrometer  arm  rotations  (abscissae)  and  the 
galvanometer  deflections  in  the  grating  spectrum  of  mica.  The  sharp 
maximum  at  A  is  the  central  image,  while  B±  is  the  selective  reflection  band 
(first-order  spectrum)  in  the  region  of  9 ,«.  For  this  part  of  the  curve 
three  reflection  surfaces  were  used,  while  to  obtain  the  part  C1?  C2  four 
reflecting  surfaces  were  employed.  The  latter  bands  are  plotted  to  a 
larger  scale  in  C\  and  C'2.  The  maxima  in  the  spectrum  to  the  left  (not 


2  3 

FIG.  20.  —  Muscovite  mica. 

plotted)  were  identical  with  the  above.  The  wave-lengths  of  the  maxima 
at  Cj,  C2  are  18.4  and  21.25  P  (Rubens  and  Nichols)  and  are  used  in  the 
calibration  curve  of  grating  No.  2,  curve  b,  fig.  19. 

RESIDUAL   RAYS   FROM   CARBONATES. 

CALCITE  (CaCO3). 
(Grating  No.  2.     K  =  0.2120  mm.     Fig.  21.) 

The  plane  cleavage  faces  of  3  large  crystals  were  used  in  this  exami- 
nation. The  spectrometer  slits  were  4  mm.  wide.  In  fig.  21  the  central 

i  It  is  a  pleasure  to  note  the  general  adoption  of  the  expression  "  residual  rays  "  for 
the  meaningless  "  reslstrahlen."  formerly  used.  The  English  vocabulary  seems  sufficiently 
complete  to  enable  writers  to  find  equivalents  to  "  ctalon,"  "  entladungsstrahlen,"  etc. 


3° 


INFRA-RED   REFLECTION   SPECTRA. 


image  A  is  plotted  to  one-fifth  the  scale  of  Bl  and  B2,  while  the  scale  of 
C'i,  C'2  and  D'^  D'2  is  five  times  that  of  B1}  B2.  The  sharp  maxima  at 
Bl  B2  are  due  to  the  complex  reflection  band  at  6.7  /*,  previously  studied. 
In  the  present  case  the  maximum  occurs  at  i°49',  which  corresponds  to 
6.78  p.  The  wave-length  of  the  maximum  at  5°  45',  D1}  D2  is  29.4  /JL  as 
determined  by  Aschkinass  (loc.  cit.).  He  located  the  maximum  at  Ct,  C2 
at  11.4  /JL  in  the  rock-salt  spectrum,  but  did  not  find  it  in  the  grating  spec- 
trum. From  his  observations  he  predicted  a  weak  band  in  the  region  of 
15  to  20  p.  The  asymmetrical  part  of  the  reflection  curve  at  +4°  indi- 
cates a  possible  band  at  14  to  16  /*,  beginning  also  in  the  transmission  curve 


2       /       o       i 

FIG.  21.  —  Calcite. 

of  calcite,  previously  studied  (see  Carnegie  Publication  No.  65,  p.  70),  where 
the  substance  becomes  entirely  opaque  at  14  /JL. 

The  band  at  11.4  /JL  deserves  further  study  because  of  the  possibility  of 
its  being  dependent  upon  the  direction  of  polarization  of  the  incident 
energy.1  In  the  transmission  curve  of  calcite  previously  studied  (see  Car- 
negie Publication  No.  65,  p.  70)  it  was  found  that  the  region  of  9  to  14  /JL 
is  quite  transparent.  A  small  absorption  band  was  found  at  11.3  /*,  which 
is  so  inconspicuous  that  it  would  appear  impossible  for  it  to  give  rise  to 
selective  reflection.  Of  all  the  substances  examined  this  is  the  first  example 
(see,  however,  nitrosodimethyl  aniline)  where  a  small  absorption  band 
apparently  coincides  with,  or  gives  rise  to,  a  reflecting  band.  In  all  other 

1  While  this  paper  is  in  press,  Nyswander  (Phys.  Rev.,  26,  p.  539,  1908),  using  polarized 
light,  found  that  the  band  at  11.3  /*  is  due  to  the  complete  absorption  (reflection)  of  the 
extraordinary  ray,  the  ordinary  ray  showing  no  trace  of  an  absorption  band  in  this  region. 
At  14.1  /*  is  a  band  due  almost  entirely  to  the  complete  absorption  of  the  ordinary  ray. 


RESIDUAL  RAYS;  CRYOLITE. 


cases  the  transmission  curves  show  complete  opacity  in  the  region  of  a 
selective  reflection  band  even  for  the  thinnest  film  yet  examined,  viz,  glass 
(see  Carnegie  Publication  No.  65,  p.  65).  Thatjhe  band  at  29.4 //  is  not 
influenced  by  the  structure  of  the  crystal  is  proven  by  the  fact  that  Asch- 
kinass  found  this  band  in  white  marble. 

MAGNESITE  (MgCO3). 
(Grating  No.  2,  fig.  22.) 

In  this  examination  three  reflecting  surfaces  of  the  massive  material 
were  used,  one  of  which  did  not  have  a  high  polish.  The  maxima  at 
Blt  Cl}  Dt  correspond  to  wave-lengths  6.7,  11.3,  and  30.7^  (5°  52'), 
respectively.  In  fig.  22  the  maxima  C\,  C'2  (=n.3//)  and  D\,  D'2 
(=30.7  /JL)  are  drawn  to  ten  times  the  scale  of  B1}  B2.  The  band  at  Dl  is 


90 

777777 


80 


70 


60 


1£  SO 

I 

£40 


30 


20 


10 


//T\ 


\ 


r 


76543          Z          I  O          I          83456          78° 

FIG.  22.  —  Magnesite. 

plotted  to  the  same  scale  as  Bl  but  only  two  reflecting  surfaces  were  used. 
The  magnesite  maxima  are  identical  with  those  of  calcite  (except  at  a 
possible  greater  wave-length,  at  30  /*).  This  would  seem  to  indicate  that 
coincidence  of  the  reflection  spectra  of  chemically  related  groups  of  sub- 
stances is  a  property  which  obtains  throughout  the  spectrum.  This  is  to 
be  expected,  but  it  seemed  of  interest  to  add  further  experimental  evidence 
to  support  this  view,  by  extending  the  observations  into  the  remote  infra- 
red. The  bands  at  29.4  /*  are  too  weak  to  illustrate  the  effect  of  molec- 
ular weight,  noticed  at  6.7  to  7.2  JJL  in  the  carbonates. 

RESIDUAL  RAYS  FROM  CRYOLITE   (3NaF-AlF3). 
(Gratings  No.  i;  slits  4  mm.;  fig.  23.) 

Reasoning  from  the  fact  that  cryolite  is  a  fluoride  of  Na  and  Al,  and 
from  the  behavior  of  fluorite  (CaF2),  it  was  hoped  to  find  this  mineral  to 


32  INFRA-RED   REFLECTION   SPECTRA. 

have  properties  similar  to  fluorite.  That  the  expectations  were  not  ful- 
filled is  perhaps  to  be  attributed  to  the  fact  that  the  specimen  was  an 
hydrated  alteration  product,  as  will  be  noticed  from  its  transmission 
spectrum  (fig.  26),  which  shows  that  the  mineral  contained  water. 

In  fig.  23  is  given  the  reflection  curve  of  cryolite,  four  large,  well-polished 
surfaces  being  used.     The  central  image  was  quite  strong.     The  first-order 


4 
cm 


-3 


-2          -/  0 

FIG.  23.  —  Cryolite. 


diffraction  band  to  the  right  and  to  the  left  is  quite  strong,  being  measured 
in  centimeters  instead  of  millimeters.  This,  of  course,  is  partly  due  to  the 
coarser  grating.  The  mean  angular  position  of  the  maximum  is  2°  43'. 
From  the  calibration  curve  the  wave-length  of  this  maximum  is  15.1  /JL, 
which  is  just  half  the  value  of  the  most  intense  band  of  fluorite. 

RESIDUAL  RAYS  FROM   OTHER  MINERALS. 
DIASPORE  [AIO(OH)]. 

In  examining  the  following  minerals  grating  No.  i  was  used  in  order 
to  obtain  an  intense  spectrum.  The  slits  were  4  mm.,  which  is  a  little 
smaller  than  those  used  by  others.  In  Carnegie  Publication  No.  65  the 
reflection  spectrum  of  an  intense  double  band  was  found  at  14  to  15  /*. 
From  its  general  trend  it  was  hoped  to  find  the  reflection  spectrum  to  con- 
tain further  bands.  Only  two  reflecting  surfaces  were  available.  With 
these,  the  maxima  at  15  /*  in  the  rock-salt  spectrum  were  verified,  but  no 
bands  were  detected  beyond  this  point. 


RESIDUAL  RAYS;  SILICATES.  33 

STIBNITE  (Sb2S3). 

Stibnite  was  previously  found  to  have  a  high  reflecting  power  up  to 
I5/*,  where  it  seemed  to  decrease.  In  the  present  examination,  using 
three  reflecting  surfaces,  this  high  reflecting  power  was  found  to  continue 
throughout  the  infra-red.  A  small  maximum  was  found  at  2  /*,  which  no 
doubt  belonged  to  the  third-order  diffraction  band.  When  using  alumi- 
num mirrors  a  similar  band  was  found  at  this  point. 

SPODUMENE  [LiAl(SiO3)2]. 

Three  large  reflecting  surfaces  were  used.  The  galvanometer  was 
perfectly  steady,  so  that  0.2  mm.  deflections  could  have  been  read,  but  no 
radiation  was  detected  beyond  that  due  to  the  reflection  bands  at  9  to  10  /* 
previously  examined  with  the  rock-salt  prism. 

CELESTITE  (SrSOj).        SELENITE  (CaSO4+2H2O). 

Three  large  cleavage  specimens  of  each  of  these  were  examined,  using 
slits  4  mm.  wide.  In  the  case  of  celestite  the  central  image  gave  a  deflec- 
tion of  50+ cm.,  but  nowhere  could  radiation  be  detected  except  in  the 
region  of  9  to  10  /JL  previously  studied.  Aschkinass  (loc.  cit.)  has  predicted 
a  band  in  the  region  of  50  /*  for  the  sulphates. 

APATITE  [Ca5F(PO4)3]. 

Three  large  ground  and  polished  surfaces  were  used  in  this  examina- 
tion. No  reflection  bands  were  observed  except  at  9  p,  where  an  exami- 
nation with  a  rock-salt  prism  showed  a  weak  (20  per  cent)  double  band. 
Apparently  the  phosphates  have  no  strong  reflection  bands  throughout  the 
infra-red  spectrum.  This  is  just  the  opposite  of  what  has  been  described 
in  Chapter  II  on  the  oxides,  where  very  intense  bands  were  frequently 

observed. 

CYANINE  (C^H^I). 

(Grating  No.  2;  slits  4  mm.) 

This  substance  was  melted  between  glass  plates,  which  were  then  split 
apart,  thus  leaving  plane  smooth  surfaces.  Three  such  films  on  glass  were 
examined.  It  was  previously  found  (see  Carnegie  Publication  No.  35, 
p.  82)  that  cyanine  is  very  opaque  at  6  to  8  /JL.  The  central  image  gave 
a  deflection  of  50+ cm.,  and  there  was  still  a  little  reflected  energy  to  be 
detected  at  25  //,  indicating  a  high  reflecting  power,  but  no  reflection  bands 
could  be  detected  throughout  the  spectrum. 

SILICATES. 

The  reflection  curve  of  mica  has  already  been  described  in  connection 
with  the  calibration  of  the  instrument.  This  examination  included  also 
serpentine,  albite,  and  microcline.  The  grating  spectrum  showed  the 
reflection  bands  at  9  //,  but  beyond  this  no  appreciable  radiation  could  be 
detected.  This,  of  course,  was  to  be  expected  for  serpentine,  for  which 


34 


INFRA-RED  REFLECTION  SPECTRA. 


the  rock-salt  prism  showed  only  weak  maxima  at  8  to  10  ft.  In  a  previous 
study  of  quartz  with  a  rock-salt  prism  a  narrow,  quite  intense  band  was 
located  at  12.5/1,  which  had  not  been  recorded  by  Rubens  and  Nichols. 
In  the  present  examination,  using  grating  No.  i,  the  maximum  was  found 
at  2°  21'  (=12.8^),  while  with  grating  No.  2  the  maximum  was  located 
at  3°  20'  (=i2.$/j).  In  table  I  are  given  the  maxima  of  the  reflection 
bands  of  the  minerals  studied. 

TABLE  I.  —  MAXIMA  OF  REFLECTION  BANDS. 


Smithsonite,  ZnCO3  

A* 
6.6< 

P 

7.o< 

0 

M 

/* 

f» 

M 

Cerussite,  PbCO3  

6.9 

7.2< 

Strontianite,  SrCO3  

6.68 

Witherite,  BaCO3  

6.70 

698 

Malachite,  CuOCO2-H2O  

6.66 

7.^ 

7.8? 

Azurite,  3  CuOa  CO2-H2O  

6.6<; 

7.72 

7.8 

Dolomite,  CaMg  (CO3)2 

6  <8 

6  o^ 

Siderite,  FeCO3  . 

66 

"•VO 
7  I 

Calcite,  CaCO3 

6d? 

6  c 

662 

C  6-85  J 

<    to    > 

16  ? 

Aragonite,  CaCOs.  . 

6  ^ 

6  7< 

7  0? 

11 

(7.0  ) 

^y*4 

Magnesite,  MgCO3.    . 

6  4.2 

6  6^ 

60 

lid 

2.O  2 

Scheelite,  CaWCu  

C«-3 

<    to 

n-3 
n  8 

^w.^ 

Zircon,  ZrSiO4  

)    LU 
(12.5 

IO  I 

12.4 
10  6 

no 

n  6 

Wulfenite,  PbMoO4  

32 

6  c 

II  7S 

1  3  i 

Rutile,  TiO2  

it  6 

Li-/O 

Cyanite,  Al2O3-SiO2  ... 

O  2. 

978 

10  28 

Corundum,  A12O3  

no 

n  8 

j-i  tf 

Topaz  (Al-F)2Si04: 
Yellow  

IO  O^ 

10  6 

IO  O 

T  T     •? 

White  

O  O 

IO  4.CJ 

II  O 

12   ? 

Spodumene,  LiAl(SiO3)2 

o  18 

O  7 

IO  d. 

Muscovite  Mica,  H2KAl3(SiO4)3.  . 
Cryolite,  3  NaF-  A1F3  

9.2 

30 

9-7 

47 

IO.2 

6  o 

18.4 

III 

21.25 

Beryl,  Be3Al2(SiO3)fl  

8  K 

92 

o  o 

IO  A. 

Quartz: 
Glass  

7  8 

8  4 

88 

Crystalline  

8  4 

n  02 

CHAPTER  V. 

ON   REGULAR   AND   DIFFUSE   REFLECTION. 

From  his  previous  work  on  the  reflecting  power  of  silicates  the  writer 
concluded  that  a  surface  like  the  earth  or  the  moon,  which  is  composed  of 
silicates,  must  be  selectively  reflecting.  From  an  examination  of  some  of 
the  subsequent  discussions  of  the  reflecting  power  of  the  moon,  it  would 
appear  that  a  matte  surface  can  not  be  selectively  reflecting.  In  fact,  the 
whole  misunderstanding  seems  to  hinge  on  this  point.  Since  the  surfaces 
of  the  minerals  examined  by  the  writer  were,  in  nearly  all  cases,  plane  and 
highly  polished,  there  was  no  "diffuse  reflection,"  which  is  an  entirely 
different  question  from  the  one  of  low,  "  practically  zero,"  reflection,  which 
the  writer  found  to  be  a  common  property  of  certain  minerals,  for  certain 
regions  of  the  infra-red  spectrum. 

When  energy  is  reflected  from  a  plane  smooth  surface,  it  is  commonly 
called  "regular"  (or,  less  accurately,  "specular")  reflection.  On  the  other 
hand,  energy  reflected  from  a  rough  surface  suffers  "diffuse"  reflection. 
The  rough  surface  is  equivalent  to  numerous  small,  plane,  reflecting  sur- 
faces, the  planes  of  which  lie  in  all  directions.  In  "diffuse  reflection"  for 
each  infinitesimal  surface,  the  ordinary  laws  of  reflection  are  obeyed  in 
full,  unless  the  linear  dimensions  of  the  reflecting  surface  or  of  the  ragosi- 
ties  or  inequalities  on  it  are  small  compared  with  the  wave-length.  How- 
ever, the  unpolished  surface  as  a  whole  destroys  all  phase  relation  between 
the  particles  in  the  reflected  wave-front,  which  is  no  longer  plane,  but 
irregular.  (See  Wood's  Optics,  p.  36.)  This  irregularity  decreases  as  the 
angle  of  incidence  increases,  so  that  for  a  given  roughness  we  get  regular 
reflection.  The  long  waves  will  be  reflected  first,  then  the  shorter  ones. 
"Smoked  glass,  which  at  perpendicular  incidence  will  show  no  image  of 
a  lamp  at  all,  will  at  nearly  grazing  incidence  give  an  image  of  surprising 
distinctness,  which  is  at  first  reddish,  becoming  white  as  the  angle  in- 
creases." (Wood's  Optics,  p.  37.) 

The  amount  of  energy  reflected  "regularly"  from  a  plane  surface  will 
depend  upon  the  reflecting  power  of  the  substance.  Now,  the  reflecting 
power  R  of  any  substance  is  related  to  its  index  of  refraction  n  and  its 
absorption  coefficient  k  by  the  equation 

For    "transparent    media,"    i.e.,    "electrical    non-conductors"    (see 

35 


30  INFRA-RED   REFLECTION   SPECTRA. 

Drude's,  also  Schuster's  Optics),  the  absorption  coefficient  is  so  low  that 
it  is  negligible,  and  the  reflecting  power  is  a  function  of  only  the  refractive 
index.  Here  the  reflecting  power  is  low,  only  4  to  6  per  cent,  and  de- 
creases with  increase  in  wave-length.  All  transparent  media  thus  far 
examined  (except  silver  chloride)  have  bands  of  selective  reflection.  In 
these  bands  the  absorption  coefficient  k  attains  high  values. 

If  k  becomes  sufficiently  large  (see  Schuster's  Optics,  Pockel's  Crys- 
tallographie,  1906),  of  the  order  unity,  the  absorption  affects  the  reflecting 
power,  and  the  heat  and  light  waves  no  longer  enter  the  substance,  but  are 
almost  totally  reflected,  as  in  metals,  whence  the  name,  "bands  of  metallic 
reflection."  For  metals,  "electrical  conductors,"  the  absorption  coefficient 


100 


I          Z         3         4         5         6         7         8  ~f~~TP         //         12 
FIG.  24.  —  Reflecting  power;  gold,  silver,  quartz,  carborundum. 


13         14/J, 


is  so  large  that  nearly  all  the  energy  for  all  (gold  and  silver  are  exceptions) 
wave-lengths  is  reflected.  In  other  words,  the  reflecting  power  of  plane 
surfaces  ("regular  reflection")  of  "transparent  media"  (electrical  non- 
conductors) will  be  low  in  all  regions  of  the  spectrum,  except  where  there 
are  bands  of  "metallic  reflection."  It  is  evident  that  the  diffuse  reflection 
from  rough  surfaces  of  transparent  media  must  also  be  selectively  reflecting. 
For  electrical  conductors  the  reflecting  power  is  high  throughout  the  infra- 
red spectrum.  The  great  dissimilarity  in  the  reflecting  power  of  these  two 
classes  of  substances  is  well  illustrated  in  fig.  24,  which  contains  the  graphs 
of  the  reflecting  power  of  gold,  silver,  quartz,  and  carborundum. 

Apropos  of  the  illustration  just  quoted,  of  the  reflection  from  smoked 
glass,  Very1  "records  that  the  percentage  of  reflected  rays,  as  measured  by 

1  Very:  Astrophys.  Jour.,  8,  p.  278,  1898. 


DIFFUSE  REFLECTION.  37 

the  bolometer,  is  especially  large  (perhaps  two  or  three  times  the  usual 
proportion)  in  the  narrow  crescent  moon  where  the  rays  suffer  a  grazing 
reflection  at  a  large  angle  of  incidence,  the  emission,  under  the  correspond- 
ing angle  of  emission  being  small."  This  is  particularly  noticeable  in  the 
early  forenoon.  Very's1  recent  discussion  gives  the  impression  that,  in  the 
moon,  "specular  reflection"  is  something  to  be  sought  for  at  the  angle  of 
reflection  with  the  sun,  at  which  the  image  of  the  sun  in  rays  of  8.5  to  10  /* 
may  be  isolated  by  using  a  screen  with  a  pinhole  aperture.  Here  "spec- 
ular reflection"  appears  to  be  used  in  the  sense  of  "regular  reflection." 
The  writer  is  not  discussing  this  limiting  case,  but  it  appears  self-evident 
that  a  matte  surface  like  a  plane  surface  can  be  selectively  reflecting,  and 
hence  that  a  surface  of  quartz,  for  example,  which,  if  smooth,  reflects  like 
a  metal  for  wave-lengths  8.5  and  9.03  /j.  and  like  a  transparent  medium 
for  all  other  wave-lengths,  must  still  reflect  selectively  when  it  is  rough.2 
Hence,  in  the  stream  of  energy  reflected  in  any  direction  the  density  will 
be  greatest  for  wave-lengths  8.5  to  9.03  /*.  Of  course  these  bands  of 
"metallic  reflection"  will  now  be  less  intense.  Since  the  energy  density 
in  every  direction  must  be  greatest  for  wave-lengths  8.5  to  10  //,  one  would 
expect  to  detect  this  difference  in  any  direction,  and  not  simply  at  the 
angle  of  regular  reflection. 

If,  then,  the  eye  were  sensitive  to  the  infra-red  quartz  would  have  a 
"surface  color"  corresponding  to  wave-lengths  8.5  and  9.03  /*.  In  speak- 
ing of  surface  color,  however,  a  sharp  distinction  must  be  made,  for  the 
reflecting  power  of  these  bands  is  as  great  as  that  of  metals,  although  the 
substance  is  a  non-conductor;  whence  one  would  expect  the  reflecting 
power  to  be  low.  In  the  case  of  transparent  non-conductors  the  surface 
color  is  due  less  to  reflection  than  to  absorption,  for  it  is  due  to  absorption 
that  the  reflected  light  is  deprived  of  some  of  its  constituents  and  becomes 
colored.  However,  on  the  long  wave-length  side  of  the  absorption  band 
the  reflecting  power  is  high,  which  contributes  to  the  color.  For  example, 
the  aniline  dyes,  such  as  fuchsine,  have  a  low  reflecting  power  (as  compared 
with  metals)  and  yet  they  possess  surface  color.  Pigments  belong  to  the 
class  of  substances  having  "body  color."  On  the  other  hand,  metals, 
such  as  gold  and  copper,  also  have  a  surface  color,  due  to  selective  absorp- 
tion. They  are  electrical  conductors,  however,  and  theoretically  would 
totally  reflect  all  radiations  for  all  wave-lengths.  This  has  been  established 

1  Very:  Astrophys.  Jour.,  24,  p.  351,  1906.     Here  the  writer  is  quoted  as  having  found 
"  that  common  minerals  reflecting  diffusively'-'  from  4  to  8  n  have  bands  of  metallic  reflection 
from  8  to  IOM. 

This  quotation  is  erroneous.  The  writer  found  that  the  reflection  was  "regular"  for  all 
wave-lengths,  but  that  from  4  to  8  /*  the  reflecting  power  is  low,  as  in  cases  of  transparent 
media  having  low  refractive  indices,  while  from  8  to  10  /x  the  reflecting  power  is  high,  like 
metals,  hence  called  "metallic  reflection." 

2  See  footnote,  p.  146,  for  experimental  evidence  supporting  these  statements. 


38  INFRA-RED  REFLECTION  SPECTRA. 

for  long  wave-lengths.1  The  reflection  curves  of  such  metals  as  gold  and 
silver,  however,  are  entirely  different  from  those  of  the  electrical  non- 
conductors (see  fig.  24).  Ifl  the  former  the  extinction  coefficient  is  always 
high,  while  in  the  latter  the  extinction  coefficient  fluctuates  through  a  great 
range.  In  quartz  the  ions  have  a  proper  period  of  undamped  electrical 
vibration  which  almost  totally  reflects  the  wave  trains  in  the  region  of 
8.5  and  9.03  /*,  while  for  the  shorter  wave-lengths  the  vibration  periods 
are  more  damped,  the  reflection  is  enormously  decreased,  and  more  of  the 
energy  is  absorbed.  In  the  case  of  a  reflecting  surface  composed  of  elec- 
trically non-conducting  material,  of  the  total  energy  falling  on  the  surface, 
the  reflected  energy  in  and  near  the  visible  spectrum  will  consist  of  that 
reflected  from  the  surface  and  that  part  which  enters  the  particles  and  is 
returned,  due  to  internal  reflection.  In  the  region  of  8  to  10  //  (for  sili- 
cates) almost  all  the  observed  energy  will  be  reflected  from  the  surface, 
and  hence  none  will  be  reemitted  due  to  internal  reflection.  As  a  whole, 
then,  what  the  writer  found  is  that  the  silicates  reflect  like  "transparent 
media"  for  all  wave-lengths  up  to  8 /i  and  like  metals  from  8.5  to  10  /*. 
In  other  words,  it  may  be  said  that  in  quartz  (SiO2)  the  silicon  ions  retain 
the  proper  period  of  undamped  electrical  vibration  which  they  would  have 
in  the  metal,  silicon,  for  wave-lengths  8.5  and  9.03  /*,  while,  for  all  other 
wave-lengths,  the  vibration  periods  are  more  damped  and  the  reflecting 
power  is  decreased. 

1  Hagen  and  Rubens:  Ann.  der  Physik,  8,  p.  i,  1902. 


PART  VI. 

INFRA-RED  TRANSMISSION  SPECTRA. 


CHAPTER  I. 

INTRODUCTION. 

With  the  accumulation  of  data  the  evidence  becomes  more  and  more 
convincing  that  there  are  no  strong  bands  of  selective  absorption  (except 
in  the  colored  glasses  given  on  a  later  page)  in  the  region  of  short  wave- 
lengths, near  the  visible  spectrum,  such  as  are  to  be  found  beyond  6  p. 
Material  suitable  for  investigation  is  obtained  with  difficulty.  The  present 
contributions  to  the  subject  of  transmission  spectra  is  rather  meager  in 
volume,  but  several  of  the  substances  show  peculiarities  worth  recording. 

The  apparatus  and  methods  of  examining  the  following  material,  con- 
sisting of  a  rock-salt  prism  and  mirror  spectrometer,  were  described  in 
Carnegie  Publication  No.  65.  Instead  of  the  radiometer,  an  improved 
Rubens 1  thermopile  was  used  to  measure  the  energy  in  the  transmission 
spectra. 

GROUP   I:    TRANSMISSION    SPECTRA   OF   VARIOUS    SOLIDS. 

MOLYBDENITE  (MoS). 

(From  Yorkes  Peninsula,  South  Australia.     Foliated,  distorted;  thickness,  £  =  0.05  and  0.31 

mm.;  fig.  25.) 

In  the  preceding  work  it  was  noticed  that  the  sulphides  are,  in  general, 
quite  transparent.  Sphalerite  (ZnS)  was  found  to  have  wide  absorption 

30 


ro 
tn 


a  . 


56789 
FIG.  25.  —  Molybdenite. 


12        13 


I4-/J. 


bands  at  3  and  15  /JL.     On  the  other  hand,  stibnite  (Sb2S3)  is  opaque  to  the 
visible  and  has  a  second  absorption  band  beyond  15  {i.     In  the  interven- 


See  Appendix  II. 


42  INFRA-RED   TRANSMISSION   SPECTRA. 

ing  part  of  the  spectrum  the  absorption  coefficient  is  very  small,  and  the  loss 
of  energy  is  due  to  the  high  reflecting  power,  which  is  about  40  per  cent. 
Molybdenite  differs  from  stibnite  in  that  it  is  very  opaque  throughout 
the  spectrum,  examined  to  15  /*.  The  two  specimens  examined  were  thin 
folia,  only  0.05  and  0.31  mm.  in  thickness  (curves  a  and  b,  fig.  25).  From 
the  greatly  decreased  transmission  of  the  thicker  piece,  it  will  be  noticed 
that  the  loss  of  energy  is  due  to  absorption  rather  than  reflection.  The 
absorption  coefficient  has  not  been  computed,  tut  an  inspection  of  the 
curves  shows  that  it  must  be  large.  By  increasing  the  thickness  six  times 
the  transmission  is  decreased  18  per  cent,  while  in  stibnite,  by  increasing 
the  thickness  five  times,  the  transmission  is  reduced  only  about  5  per  cent, 
in  the  region  of  general  absorption. 


90% 


O'/  2345675 

FIG.  26.  —  Kuntzite  (a);  Cryolite  (6);  Carborundum  (c);  Nitrocellulose. 


KUNTZITE  [LiAl(SiO3)2]. 

(From  Pala,  California.     Cleavage  piece,  parallel  to  m;  transparent;  ^  =  1.05  mm. 

Curve  a,  fig.  26.) 

This  mineral,  which  is  violet  spodumene,  is  obtainable  in  large  speci- 
mens, hence  it  was  of  interest  to  determine  its  transparency  to  infra-red 
radiation.  It  is  lacking  in  the  small  band  of  SiO2  at  2.95  /*  and  is  more 
opaque  than  quartz  in  the  region  of  i  /*,  so  that  it  would  not  be  more 
serviceable  than  the  latter  in  optical  work. 


VARIOUS   SOLIDS.  43 

CARBORUNDUM  (SiC). 
(Artificial  product;  ^  =  1.27  mm.     Curve  c,  fig.  26.) 

The  specimen  examined  was  a  flat  crystal  which  transmitted  blue  light, 
and  reflected  yellowish-green  light.  It  has  remarkable  optical  properties,1 
especially  a  high  value  for  the  refractive  index.  It  is  opaque  in  the  ultra- 
violet (Jewell)  and  beyond  2  //  in  the  infra-red.  In  this  respect  the  trans- 
mission curve  c,  fig.  26,  is  very  remarkable  as  compared  with  all  the  other 
substances  examined,  of  which  only  one,  viz,  mellite,  is  as  opaque  to  infra- 
red rays.  The  reflection  curve  was  examined  previously;  it  also  shows 
remarkable  properties. 

CRYOLITE  foNaF  •  A1F3). 
(From  Ivigtut,  Greenland.     Massive,  semitranslucent;  £  =  2.3  mm.     Curve  &,  fig.  26.) 

From  the  composition  of  the  material  it  was  hoped  to  find  this  to  be 
more  transparent  to  heat  rays  than  usually  has  been  the  case  with  minerals. 
Possibly  a  pure  crystal  of  cryolite  would  be  different.  From  the  absorp- 
tion bands  (curve  b,  fig.  26)  it  will  be  noticed  that  this  specimen  contained 
water  of  crystallization. 

Thomsenolite  (NaLiAlF6  +  H2O),  previously  examined,  is  an  alteration 
product  of  cryolite,  and  from  the  similarity  of  the  transmission  curves  it 
would  appear  that  this  sample  of  cryolite  had  already  undergone  some 
alteration  into  the  hydrated  mineral. 

NITROCELLULOSE  (TRANSPARENT  "CELLULOID"), 
(Curve  d,  fig.  26;  2=0.138  mm.) 

Transparent  celluloid  is  a  mixture  of  nitrocellulose  [C12H16(NO2)4O10] 
and  camphor  (C10H16O).  The  transmission  curve  which  was  obtained 
with  a  fluorite  prism  and  bolometer  is  conspicuous  for  two  regions  of 
great  absorption,  with  maxima  at  3  //,  3.43  //,  and  6  fi. 

Camphor  is  a  very  complex  carbohydrate.  Its  alcoholic  properties, 
OH  groups,  should  cause  an  absorption  band  at  3  fi  (2.95  /*  in  Carnegie 
Publication  No.  35,  p.  58,  for  rock-salt  prism).  Its  ketone  properties, 
CO  groups  (perhaps  aldehyde  properties,  CHO  groups),  should  cause  a 
strong  band  at  6  /*.  Excellent  examples  of  OH  groups  are  (see  Carnegie 
Publication  No.  35,  p.  108)  carvacrol  and  thymol,  of  CHO  groups  are 
benzaldehyde,  cuminol,  eucalyptol,  colophonium,  and  Venice  turpentine, 
and  of  both  these  groups  is  terpineol.  The  band  at  3.43  //  is  characteristic 
of  the  CH3-groups. 

Excepting  in  intensity,  the  transmission  curve  is  similar  to  that  of  col- 
lodium  (Carnegie  Publication  No.  65,  p.  60),  which  is  also  a  complex 
compound  of  cellulose.  It  seems  rather  remarkable  that  the  characteris- 
tic vibration  period  of  these  groups  of  atoms  is  not  affected  by  the  com- 
plexity of  the  compound. 

1  Jewell:  Physical  Review,  24,  p.  339,  1907. 


44 


INFRA-RED   TRANSMISSION   SPECTRA. 


SIDERITE  (FeCO3). 

(From  Allevard;  France.     Cleavage  piece;  £  =  1.2  mm.;  grayish  color;  translucent 
to  transparent.     Fig.  27.) 

This  specimen  was  examined  in  connection  with  the  carbonates  pre- 
viously studied.  The  complex  band,  with  maxima  at  3.4,  3.9,  and  4.6  /*, 
is  in  common  with  the  other  carbonates.  In  a  very  thin  section  of  calcite 


25% 


234 
FIG.  27.  —  Siderite. 

(CaCO3)  this  region  was  resolved  into  sharp  bands,  having  maxima  at 
3.44,  3.93,  and  4.6  //.  This  sample  furnishes  additional  proof  that  the 
large  groups  of  chemically  homologous  compounds  have  similar  absorption 
spectra. 

WlTLFENITE  (PbMoO4). 

(From  Red  Cloud  Mine,  Yuma  County,  Arizona.     Deep-yellow  color;  t  =  2.i  mm. 

Curve  a,  fig.  28.) 

This  mineral  is  of  interest  in  connection  with  a  study  of  the  oxides, 
e.  g.,  TiO2,  SiO2,  etc.     The  transmission  curve  a,  fig.   24,  shows  wide 


3U 

25 

C20 
.9 
w 

'e  /s 

W 

§'. 

5 

>*k 

S6 

1 

i 
\ 

i 
i 
» 

<? 

i 
i 
—  >« 

f 

b 

'% 

r 

f 

""\ 

\ 

*X 

^V 

7 

i_X-JT- 

~~s~-+ 

li 

\ 

i 

u 

\ 

\ 
\ 

A 

& 

V 

r 

V" 

--« 

/  234-567 

FIG.  28.  —  Wulfenite  (a) ;  Phosphorus. 


10/JL 


absorption  bands  at  3.15  and  6.5^,  and  complete  opacity  beyond  9 //, 
where  there  is  a  band  of  metallic  reflection. 


VARIOUS    SOLIDS. 


45 


PHOSPHORUS  (P). 
(Solid  film  melted  between  plates  of  fluorite.    /  =  o.8  mm.  (?).     Curve  &,  fig.  28.) 

Phosphorus  was  examined  in  order  to  answer^an  inquiry  whether  it 
could  be  used  for  prisms  contained  in  vessels  with  quartz  or  fluorite 
windows.  The  yellow  stick  phosphorus  was  used.  It  was  melted  under 
water  (at  44°  C.),  poured  on  a  glass  plate,  and  pressed  into  a  thin  sheet 
by  means  of  a  second  plate  of  glass.  This  thin  sheet  was  then  placed 
between  two  plates  of  fluorite  and  the  edges  covered  with  soft  wax.  As 
will  be  noticed  in  curve  b,  fig.  28,  the  transmission  curve  shows  bands  due 
to  water  which  adhered  to  the  phosphorus.  The  latter  melted  during  the 
latter  part  of  the  examination.  The  transmission  curve  includes  the 
fluorite  plates,  which  were  not  clear,  and  hence  increase  the  observed 
absorption.  QuARTZ  (SiOs)> 

(2=3  mm.     Curve  &,  fig.  29.) 

The  sample  examined  was  used  as  a  window  on  the  spark-tube  in  the 

emission  spectra  of  metals  in  hydrogen,  Part  VII.     The  curve  shows  that 

emission  lines,  if  of  appreciable  intensity,  would  be  transmitted  out  to  4  /*. 

ioo%\ — 


90 


80 


70 


60 


I40 


\ 


V 


V 


2  3  *  5  6  7  8 

FIG.  29.  —  Glass  (a);  Quartz  (6);  Fluorite. 


10 


/2/i 


GLASS. 
(Thickness  =  0.7 5  mm.     Curve  a,  fig.  29.) 

This  sample  was  examined  in  connection  with  prospective  work  on 
radiation  from  incandescent  lamps.  The  transmission  is  uniform  (80  per 
cent)  out  to  2  to  2.4  /*.  From  this  it  will  be  seen  that  the  maximum  of 
the  energy  spectrum  of  the  filament  will  not  be  affected  by  the  glass  bulb. 


46 


INFRA-RED  TRANSMISSION  SPECTRA. 


FLUOEITE  (CaF). 

(Curve  c,  *~2.a8  mm.;  curve  d,  *«io  mm.;  curve  e,  *=«i.84  mm.;  curve/,  <=3-85  mm. 
light-green  color;  fig.  29.) 

On  account  of  the  increased  scarcity  of  this  material,  it  is  of  interest 
to  determine  the  effect  of  inclusions  upon  the  transmission.  In  fig.  29, 
curve  c  was  a  perfectly  clear  specimen;  while  curve  e  contained  numerous 
small  inclusions,  or,  perhaps  more  exactly,  small  cleavage  planes.  The 
latter  when  held  at  a  distance  of  30  to  50  cm.  from  the  eye  appeared  quite 
blurred.  Nevertheless,  throughout  the  infra-red  the  loss  of  energy  is  only 
about  2  per  cent  greater  than  that  of  clear  fluorite.  This  is  to  be  expected ; 
for  each  cleavage  plane  reflects  some  of  the  light,  and  the  magnitude  of 
this  reflection,  since  it  depends  upon  the  refractive  index,  decreases  with 
increase  in  wave-length.  Curve  /  shows  the  transmission  of  a  specimen 
of  light  yellowish-green  fluorite  (see  Carnegie  Publication,  No.  65,  p.  69, 
for  another  example)  which  has  an  absorption  band  at  i.4/£,  hence  not 
suitable  for  a  prism.  This  specimen  was  free  from  inclusions. 

CARBON  (C). 
(Curves  a,  b,  c,  lampblack;  curve  d,  diamond;  fig.  30.) 

The  commonest  and  most  conspicuous  example  of  the  effect  of  structure 
upon  absorption  is  to  be  found  in  carbon  in  the  form  of  lampblack  and 

too% 


90 


3  456  7  8  B/JL 

FIG.  30.  —  Carbon  (a),  (i),  (c);  Diamond. 


of  diamond.  Since  the  observations  on  these  two  substances  are  published 
in  separate  and  rather  isolated  places  they  are  incorporated  here  for  com- 
pleteness of  illustration  of  the  effect  of  structure.  The  transmission  curves 


VARIOUS   SOLUTIONS.  47 

a,  b,  c,  thickness  0.038,  0.023,  anc^  o-OQQ  mm.",  respectively,  of  lampblack 
are  due  to  Angstrom.1  The  transmission  curve  of  diamond  is  due  to 
Julius.2  The  lampblack  is  opaque  to  the  visible  and  shows  an  increase 
in  transmission  with  wave-length.  The  diamond  crystal  is  transparent  to 
the  visible  and  has  well-defined  absorption  bands  at  3,  4,  and  5  /*  and 
complete  opacity  beyond  8  /*.  A  band  of  metallic  reflection  is  predicted 
at  12  ft,  which  happens  to  coincide  with  the  band  of  carborundum. 

It  seems  rather  remarkable  that  diamond  should  have  one  of  its  prin- 
cipal absorption  bands  in  the  region  where  carbohydrates  have  a  wide 
band  of  great  transparency. 

GROUP   II:    TRANSMISSION    SPECTRA   OF  VARIOUS   SOLUTIONS. 

The  data  presented  here  were  obtained  several  years  ago  (but  not  pub- 
lished) in  connection  with  an  investigation  of  methods  of  measuring  radi- 
ant efficiencies.3  One  of  the  methods  of  measuring  the  efficiency  of  an 
illuminant,  i.  e.y  the  ratio  of  the  light  emitted  to  the  total  radiation,  is  to 
absorb  the  infra-red  by  means  of  a  water  cell.  The  idea  persists  even  to 
this  day  that  a  water  solution  of  potassium  alum  absorbs  more  heat  than 
does  clear  water,  although  Donath  and  others  demonstrated,  long  ago,  that 
this  is  not  the  case;  the  data  herewith  presented  is  further  proof  of  this 
fallacy.  During  the  present  year  (Phys.  Zeitschrift,  1907)  measurements 
of  radiant  efficiencies  have  been  made  using  a  water  solution  of  ammo- 
nium-iron alum,  and  it  was  claimed  that  the  solution  was  more  opaque 
than  pure  water.  A  transmission  curve  of  this  substance  will  be  shown 
presently  which  indicates  a  greater  opacity  than  water.  The  iron  alum 
oxidizes  readily  and  it  is  difficult  to  keep  the  solution  clear. 

The  problem  was  recently  presented  to  the  writer  by  the  Astrophysical 
Observatory  to  suggest  a  substance  (see  asphaltum)  that  absorbs  all  the  visi- 
ble or  all  the  infra-red  energy,  the  dividing  line  being  0.76  /*.  Such  a  sub- 
stance would  of  course  be  an  ideal  energy  filter;  but  no  such  substance  is 
known.  Indeed,  from  all  the  substances  examined,  it  appears  that  none 
(at  least  none  of  the  common  ones)  have  large  absorption  bands  near  the 
visible.  Beryl 4  is  recorded  as  having  a  large  band  at  0.89  //,  when  ex- 
amined in  polarized  light.  Cyanine  would  be  a  fairly  good  material  for 
absorbing  the  visible,  but  it  is  very  opaque  in  the  region  of  6  to  8  /*.  Io- 
dine would  be  much  better  material  for  absorbing  the  visible,  and  trans- 
mitting the  infra-red.  Its  absorption  band,  at  about  7.3  //,  is  very  weak. 
The  absorption  band  of  iodine  in  the  visible  spectrum  corresponds  closely 
with  the  sensibility  curve  of  the  eye.  Since  we  are  concerned  only  with  the 
light  that  affects  the  eye,  the  fairest  rating  of  efficiencies  would  be  to  com- 

1  Angstrom:  Wied.  Annalen,  36,  p.  717,  1889. 

2  Julius:  Konigkl.  Akad.  Wiss.  Amsterdam,  Deel  I,  No.  i,  1892. 
•Nichols  &  Coblentz:  Phys.  Rev.,  17,  p.  267,  1903. 

4  Kb'nigsberger:  Ann.  der  Phys.  (3),  61,  p.  687,  1897. 


4»  INFRA-RED   TRANSMISSION  SPECTRA. 

pare  the  energy  that  affects  the  normal  eye  to  the  total  energy.  This  would 
lower  the  present  efficiencies  of  sources  that  are  rich  in  the  red,  which 
color  affects  the  eye  but  little. 

The  use  of  iodine  in  solution  is  prohibited  by  the  selective  absorption 
of  heat  rays  by  all  known  solvents.  On  the  other  hand,  solid  iodine 
evaporates  readily,  so  that  it  would  be  impossible  to  keep  a  solid  film  of 
uniform  thickness  for  any  great  length  of  time. 

ASPHALTUM. 

(Curve  c,  t  =  o.i  mm,;  0  =  0.0005  min-;  6  =  0.03  mm.;  ^  =  0.005  mm.;  fig.  31.     For  the  complete 
curve,  to  14  /*,  see  Carnegie  Publication  No.  35,  p.  75,  and  fig,  44,  p.  198.) 

The  next  best  substance  to  iodine  for  absorbing  the  visible  and  trans- 
mitting the  infra-red  is  asphaltum,  which  is  a  solid  hydrocarbon,  previously 
examined.  This  substance  can  be  formed  into  films  of  any  desired  thick- 
ness. In  fig.  31  curves  a  and  b  are  due  to  Nichols.1  From  the  curves  it 


100% 


§40 


30 


20 


10 


7 


\ 


\ 


Z  3/J.  4 

FIG.  31.  —  Asphaltum. 


7JU. 


will  be  seen  that  a  thickness  can  be  selected  which  fulfills  the  conditions 
of  complete  transparency  beyond  0.7/1  better  than  any  other  common 
substance.  Even  the  thick  film  (curve  c,  t=o.i  mm.),  which  transmitted 
a  trace  of  red,  shows  great  transparency,  after  passing  beyond  the  effect 
of  the  absorption  band  in  the  visible.  A  film  absorbing  up  to  0.65  or  0.7  /j. 
would  be  practically  transparent  in  the  infra-red  (see  Carnegie  Publica- 
tion No.  35,  p.  17),  where,  for  a  very  thin  film  of  water,  the  small  absorption 
band  at  4.75  /*,  similar  in  intensity  to  those  of  asphaltum,  has  entirely 
disappeared.  By  using  a  clear  rock-salt  plate,  covered  on  both  sides  to 
prevent  the  action  of  moisture,  a  fair  standard  could  be  produced.  The 


E.  L.  Nichols:  Phys.  Rev.,  14,  204,  1902. 


VARIOUS   SOLUTIONS. 


49 


asphaltum  may  be  applied  to  the  rock-salt  as  a  thin  varnish,  the  volatile 
parts  of  which  evaporate  in  a  short  time. 

Asphaltum  varnish,  applied  in  an  extremely  thiri^coating,  forms  an  excel- 
lent protecting  surface  for  radiometer  or  radiomicrometer  windows  made 
of  rock-salt.  It  might  also  be  used  advantageously  as  a  covering  for 
rock-salt  prisms  to  protect  them  from  moisture.  The  pair  of  films  of 
asphaltum  to  be  used  for  such  purposes  can  be  made  extremely  thin,  and 
are  almost  transparent,  as  shown  in  fig.  31,  curve  d,  which  was  of  a  light 
amber  color  showing  but  little  absorption  in  the  visible  spectrum.  The 
film  was  prepared  from  commercial  asphaltum  varnish,  which  is  a  very 
thick  solution,  by  diluting  it  with  gasoline.  This  precipitates  the  very 
insoluble  constituents,  which  apparently  are  simply  held  in  suspension, 
and  by  experimenting  on  the  amount  of  gasoline  to  be  added,  a  film  or 
pair  of  films  of  any  desired  thickness  may  be  produced  by  a  single  dipping 
of  the  plate  or  prism.  Of  course,  a  thicker  film  may  be  formed  by  using 
a  thicker  solution,  or  by  applying  several  coats  of  the  thin  solution.  By 
filtering  the  dilute  solution  through  cotton  wool  or  through  filter  paper 
the  black  insoluble  asphaltum  particles  are  removed.  The  dried  film  is 
then  homogeneous  and  perfectly  free  from  even  traces  of  black  particles. 
The  gasoline  must  be  quite  free  from  moisture,  otherwise  there  is  likely  to 
be  a  slight  action  on  the  rock-salt  surfaces,  and  the  dried  film  will  not  be 
so  homogeneous  as  that  obtainable  on  a  glass  plate.  It  may  be  possible 
to  apply  a  homogeneous  film  of  paraffin  or  of  ozokerite,  which  is  white, 
and  which,  on  account  of  its  low  refractive  index,  will  reflect  less  than  a 
rock-salt  surface;  but  neither  of  these  substances  are  as  satisfactory  as 
asphaltum  purified,  as  indicated  above.  Such  surface  coverings  for  hydro- 
scopic  substances  are,  of  course,  useful  only  where  the  radiometer  and 
prism  are  used  in  making  relative  comparisons,  i.  e.,  it  is  not  adapted  to 
making  spectral  energy  measurements  of  sources  of  radiation,  where  the 
absorption  of  the  apparatus  and  the  atmosphere  must  be  eliminated.1 
Curve  d  is  for  two  films  on  a  plate  of  glass,  0.175  mm-  thick.  The  values 
are  obtained  by  dividing  the  observed  transmission,  when  the  glass  was 
coated  with  asphaltum,  by  the  observed  transmission  through  the  clear 
glass.  The  fact  that  the  transmission  is  greater  than  100  per  cent  is  due 
to  the  higher  reflecting  power  of  glass,  for  which  no  correction  was  made. 

1  Since  writing  this,  Dr.  A.  Trowbridge,  at  the  Washington  Meeting  of  the  American 
Physical  Society,  April,  1908,  described  the  transmission  of  extremely  thin  films  of  collodium 
(Carnegie  Publication  No.  65,  p.  60,  fig.  46)  and  their  application  to  rock-salt  as  a  protecting 
surface.  For  the  region  of  the  spectrum  up  to  6  /*  there  are  no  prominent  absorption  bands, 
and  the  small  ones  at  3.3/4  have  almost  entirely  disappeared  in  the  thinnest  films,  which  are 
of  the  order  of  a  light-wave  in  thickness.  Collodium  is  said  to  be  slightly  hydroscopic,  but 
whether  it  is  sufficiently  so  to  become  separated  from  the  rock-salt  is  unknown.  At  this 
meeting  it  was  learned  that  at  the  Astrophysical  Observatory  it  is  proposed  to  use  asphaltum 
varnish  to  protect  the  large  rock-salt  prism  which  is  about  18  cm.  high. 


50  INFRA-RED   TRANSMISSION   SPECTRA. 

ALUM  SOLUTION. 
(Saturated  solutions  at  o°  C.;  cell  i  cm.  thick;  fig.  32.) 

In  fig.  32  are  given  the  transmission  spectra  of  water  (o-o-o-o)  and 
of  solutions  of  the  alums  of  potassium  (x-x-x),  ammonium  (•_.-•_,),  and 
of  ammonium-iron.  The  curves  are  due  to  E.  F.  Nichols1  and  are  in- 
cluded here  because  of  their  bearing  upon  the  present  data.  The  trans- 
mission curves  of  the  solutions  of  potassium  and  of  ammonium  alum  are 
identical  with  that  of  water.  The  ammonium-iron  alum  [(NH4)2Fe2(SO4)7] 

80 


60 


.I50 


I" 


20 


\\ 


5  .7  9  /./  1.4/J. 

FIG.  32.  —  Water  o-o-o;  Potassium  alum 
X-x-x;  ammonium  alum  •-•-.-•;  ammo- 
nium iron  alum  (lowest  curve). 

shows  greater  opacity,  which  from  the  trend  of  the  curve  appears  to  extend 
into  the  visible  spectrum,  due  perhaps  to  a  trace  of  iron  oxide  which  is 
avoided  with  difficulty.  The  transmission  curve  of  a  clear  plate  of  alum 
4  mm.  in  thickness  is  shown  in  curve  a,  fig.  33. 

BORAX  AND  POTASSIUM  PERMANGANATE. 
(Cell  i  cm.  thick;  glass  walls;  fig.  33.) 

In  fig.  33  are  given  the  transmission  curves  b  of  borax  (Na2Br4O7)  and 
curve  d  of  potassium  permanganate  (KMnOJ.  The  latter  solution  was 
not  saturated.  The  results  show  that  there  is  no  increased  absorption  of 
the  solution  over  that  of  pure  water,  except  in  permanganate,  which,  of 
course,  is  almost  opaque  to  the  visible. 

LANTHANUM  NITRATE  (LaNO3);  DIDYMIUM  NITRATE  [Pr,Nd(NO3)3]; 

SULPHURIC  ACID  (H2SO4);  LIQUID  GLASS  (Na2SiO3). 
(Cell  i  cm.     Concentration  of  nitrates  unknown.     Fig.  34.) 

In  fig.  34  are  given  the  transmission  curves,  a  of  sulphuric  acid,  b  of 
lanthanum  nitrate  (solution),  c  of  didymium  nitrate  (solution),  and  d  of 


E.  F.  Nichols:  Phys.  Rev.,  i,  p.  i,  1896. 


COLLOIDAL  METALS. 


liquid  glass.    The  sulphuric  acid  is  more  transparent  than  water,  while 
the  didymium  nitrate  has  a  sharp  absorption  band  at  0.76/1.    The  latter 


90% 
80 
70 


.260 
H 
.« 

W50 


.5 


.7 


.9 


1.3         I.S 


I.7JUL 


FIG.  33.  —  Potassium  alum  (a);  Borax  solution  (ft); 
Water  (c);  Potassium  permanganate  solution. 


FIG.  34.  —  Sulphuric  acid  (c);  Lanthanum  nitrate  (6); 
Didymium  nitrate  (c);  Liquid  glass. 


have  absorption  bands  in  the  visible,  so  that  they  would  not  be  useful  for 
efficiency  work.     The  same  is  true  of  liquid  glass. 

CHLORIDES  OF  THE  YTTRIUM  GROUP  (Sc,Yt,  La,Yb);  NEODYMIUM  NITRATE  [Nd(NOs)s]. 
(Cell  i  cm.     Concentration  unknown.     Fig.  35.) 

These  solutions,  like  the  preceding  ones,  were  obtained  from  the  Chem- 
ical Laboratory  of  Cornell  University,  and  the  concentration  was  unknown. 
They  were  examined  to  learn  whether  the  sharp  absorption  bands  in  the 
visible  are  also  to  be  found  in  the  infra-red.  It  will  be  noticed  that  the 
didymium  nitrate  (curve  b,  fig.  35)  has  two  bands  at  0.78  and  0.98 //, 
while  the  yttrium  group  of  chlorides  have  a  band  in  common  with  didym- 
ium at  0.98  fi. 

GROUP   III:    TRANSMISSION    SPECTRA   OF   COLLOIDAL   METALS. 

The  metals  and  non-metals  present  two  distinct  types  of  absorption,  of 
reflection  and  of  emission  spectra.  The  metals  (electrical  conductors) ,  even 
in  very  thin  films,  are  extremely  opaque  to  all  radiations  throughout  the 
spectrum,  except  gold,  silver,  and  copper,  which  have  narrow  transparent 
bands  at  0.32,  0.5,  and  0.6  //,  respectively.1  The  opacity  is  really  due  to 


1  Hagen  &  Rubens:  Ann.  der  Phys.,  8,  p.  432,  1902;  Javal:  Ann.  de  Chim.  et  Phys.  (8), 
4,  p.  137,  1895. 


80% 


70 


.<240 

B 

§30 


20 


10 


52  INFRA-RED    TRANSMISSION    SPECTRA. 

their  high  reflecting  power,  which  is  uniform  throughout  the  infra-red. 
Their  emission  (arc  and  spark)  spectra  consist  of  numerous  fine  lines, 
which  occur  throughout  the  visible  and  ultra-violet  part  of  the  spectrum. 

No  strong  lines  have  yet  been  found  in 
the  infra-red,  except  in  the  alkali  metals. 
On  the  other  hand,  the  non-metals  (insu- 
lators) have  absorption  bands  throughout 
the  spectrum.  Their  reflecting  power  is 
low,  and  in  some  compounds  is  highly 
selective  in  the  infra-red.  They  have 
emission  lines  which  extend  far  into  the 
infra-red.  The  elements  on  the  border 
line  between  metals  and  non-metals  are 
of  peculiar  interest,  and  the  reflecting 
power  of  their  compounds  deserves  further 
study.  A  notable  example  is  silicon, 
which  from  the  known  properties  of  non- 
metals  would  be  expected  to  have  a  uni- 
formly low  reflecting  power  throughout  the 
FIG.  35.  — chlorides  of  Yttrium  Group  (a);  infra-red ;  in  the  form  of  a  compound, 

Neodymium  nitrate.  o-,^       ,1  n 

SiO2,  the  reflecting  power  at  8.5  and  9  /* 

attains  a  value  as  high  as  that  of  metals.  The  elements  iodine,  selenium, 
and  sulphur  have  a  uniformly  low  reflecting  power.  It  would  be  inter- 
esting to  learn  whether  only  the  compounds  of  non-metals  have  bands 
of  selective  reflection  in  the  infra-red;  all  of  our  present  data  indicate 
that  only  in  a  compound  (cf.  SiO2)  do  the  ions  of  non-metals  attain  a 
freedom  such  as  obtains  in  metals. 

SILVER. 
(Colloidal  film,  on  glass.     Curve  b,  fig.  36.) 

The  film  examined  was  made1  by  Prof.  R.  W.  Wood,  and  was  of  a 
deep  ruby-red  color.  In  fig.  36,  curve  b,  is  given  the  transmission  curve 
of  a  thin  uniform  film  of  colloidal  silver,  deposited  on  glass.  The  trans- 
mission begins  in  the  red,  and  increases  uniformly  throughout  the  infra-red 
to  4  //,  beyond  which  it  was  not  possible  to  extend  the  observations  on 
account  of  the  opacity  of  the  glass.  This  is  exactly  the  reverse  effect 
observed  with  a  metallic  film  (see  fig.  24),  where  the  transparency  lies  in 
the  violet  and  the  opacity  extends  throughout  the  infra-red.  The  film  of 
colloidal  silver  behaves  like  a  turbid  medium,  which  has  the  property  of 
increasing  in  transparency  with  wave-length.  This  is  in  agreement  with 
Garnett,2  who  concludes  from  the  optical  properties  of  Gary  Lea's  so-called 
solutions  of  allotropic  silver  that  they  consist  of  small  spheres  of  silver,  i.e., 

1  Gary  Lea's  Allotropic  Silver,  Amer.  Jour.  Sci.j  vol.  37,  p.  746,  1889. 

2  Garnett:  Proc.  Roy.  Soc.  (A),  76,  p.  370,  1905-06;  Phil.  Trans.,  A,  pp.  385-420,  1904. 


COLLOIDAL   METALS. 


53 


normal  silver  in  a  finely  divided  state,  little  if  any  being  in  true  solution 
(molecularly  subdivided).  He  further  concludes  that  the  color  of  ruby 
gold  glass  is  due  primarily  to  the  presence  of  small  spheres  of  gold;  the 
irregular  blue  and  purple  colors,  sometimes  exhibited  by  gold  glass,  are 
then  explained  by  the  presence  of  crystallites  (caused  by  the  coagulation 
of  gold  spheres)  which  reflect  but  do  not  transmit  red  light. 


100% 


90 


80 


70 


§40 


30 


20 


10 


Colloidal) 


u  (rrielal) 


Ag(Colloidal) 


0  .5  /  3  4/* 

FIG.  36.  —  Gold;  Silver;  Nickel. 

In  view  of  the  fact  that  colloidal  metals  appear  to  behave  exactly  the 
reverse  of  films  of  a  metal  in  the  normal  state,  the  subject  deserves  further 

consideration. 

GOLD  AND  NICKEL. 

In  fig.  36,  curve  a  gives  the  transmission  (Hagen  and  Rubens,  loc.  cit.) 
of  a  metallic  film  of  gold,  which  has  the  well-known  transmission  band  in 
the  yellowish-green  part  of  the  spectrum.  Curves  c  and  d  give  the  trans- 
mission of  suspensions,  in  water,  of  colloidal  gold  and  nickel,  respectively, 
studied  photometrically  by  Ehrenhaft.1  The  gold  transmitted  red,  while 
nickel  (also  Pt  and  Co)  have  a  brown  color  by  transmitted  light.  As  in 
the  present  work,  the  behavior  of  the  colloidal  material  is  exactly  the 
reverse  of  the  normal  metal  film,  thus  placing  it  under  the  class  known  as 
non-metals  or  "  insulators." 


1  Ehrenhaft:  Ann.  der  Phys.,  II,  p.  489,  1903. 


54 


INFRA-RED  TRANSMISSION  SPECTRA. 


GROUP   IV:   TRANSMISSION   SPECTRA   OF   COLORED   GLASSES. 

COBALT  GLASS. 
(Fig.  37.     Thickness,  2.43  mm.) 

Colored  glasses  have  been  but  little  studied.  Nichols1  examined  cobalt 
glass,  and  found  an  absorption  band  at  1.4  p.  This  is  one  of  the  few 
substances  having  a  prominent  band  near  the  visible  spectrum.  Garnett 
(loc.  cit.)  concludes  that  the  deep  blue  color  of  cobalt  glass  can  not  be 
due  to  small  diffused  spheres  of  metallic  cobalt,  which  would  give  a  reddish 
color  to  transmitted  light,  but  that  the  metal  is  in  the  form  of  discrete 
molecules  (amorphous). 

For  the  present  work  a  fluorite  prism  and  bolometer  were  used  (see 
Appendix  II). 


80% 


2  3 

FIG.  37.  —  Cobalt  blue  glass. 

In  fig.  37  is  given  the  transmission  curve  of  a  cobalt  blue  glass  which 
showed  three  weak  absorption  bands  in  the  visible  spectrum.  In  the 
infra-red  there  is  a  larger  absorption  band  at  1.5  /*,  a  smaller  band  at 
2.1  /z,  and  second  large  band  at  3.5  p.  The  small  band  at  3  /z  is  due  to 
the  glass  itself,  and  is  also  found  in  quartz.  Beyond  4  //  the  opacity  is 
due  to  the  glass,  as  found  in  clear  specimens. 

The  behavior  of  this  glass  is  entirely  different  from  the  red  glasses  to 
be  noticed  on  a  following  page.  It  is  not  like  an  optically  turbid  medium, 
unless  we  consider  it  to  have  more  than  one  region  of  selective  absorption, 
which  is  in  line  with  the  conclusion  arrived  at  by  Garnett  (loc.  cit.). 

1  Nichols,  E.  F.:  Phys.  Rev.,  I,  p.  i,  1893. 


COLORED   GLASSES. 


55 


BLUE-VIOLET  GLASS. 
(Schott's1  No.  F  3086;  £  =  2.58  mm.) 

In  fig.  38  is  given  the  transmission  of  a  plate  of  blue- violet  glass.  There 
is  a  large  absorption  band  at  0.5  to  2  /*,  followed  ^y  smaller  bands  at  2.1 
and  3.5  fi.  In  the  deep  infra-red  such  a  band  would  cause  selective  re- 
flection. In  the  present  case  the  absorption  band  is  due  to  the  metal 
behaving  somewhat  like  an  optically  turbid  medium.  Using  a  pocket 
spectroscope  close  to  a  Nernst  glower,  it  was  found  that  a  trace  of  yellow 


30 


020 


V) 

t 
f=  '0 


2  3 

FIG.  38.  —  Blue-violet  glass. 


5/JL 


at  0.56  fj.  and  of  red  at  0.707  /*  was  transmitted,  the  intensity  of  the  maxima 
being  of  the  order  of  o.oi  per  cent.  In  the  violet  the  transmission  was 
30  per  cent.  This  region  was  studied  with  a  spectrophotometer.  Ang- 
strom has  very  ingeniously  applied  this  glass  in  measuring  the  violet  radia- 
tion from  the  sun;  using  an  absorption  cell  of  water,  i  cm.  thick,  in  addition 
to  this  glass  plate,  all  the  radiation  beyond  0.5  is  absorbed. 

GREEN  GLASS. 
(Schott's  copper  oxide,  No.  431  III.     Fig.  39.     Thickness  2=3. 43  mm.) 

The  specimen  examined  showed  an  absorption  band  in  the  violet  and 
a  second  one  in  the  red.  There  appears  to  be  a  small  band  at  2  //,  beyond 
which  point  there  is  the  usual  absorption  of  uncolored  glass.  The  band 
at  2.95  /*,  found  in  silicates,  e.g.,  mica,  seems  to  be  intensified  by  the  pres- 
ence of  coloring  substance,  while  the  transmission  seems  to  be  terminated 
at  a  shorter  wave-length  (also  due  to  the  coloring  substance)  than  would 
be  found  in  a  clear  plate  of  glass  of  the  same  thickness. 

RUBY  GLASS. 

(Fig.  40.     Curve  a,  Schott's  monochromatic  red,  No.  2745;  ^=3.18  mm. 
Curve  6,  impure  red;  £  =  2.48  mm.) 

The  monochromatic  red  glass  examined  is  used  in  the  Holborn  and 
Kurlbaum  pyrometer.  The  second  sample,  curve  b,  transmitted  impure 


1  See  Zsigmondy,  Zs.  fur  Instrk.,  21,  p.  97,  1901. 


INFRA-RED   TRANSMISSION   SPECTRA. 


red;  it  was  taken  from  a  Le  Chatelier  pyrometer  and  is  described  as  a 
copper  oxide  glass.  Although  thinner,  the  second  sample  is  the  more 
opaque  in  the  region  of  i  to  2.5  ,«.  The  transmissivity  of  the  monochro- 
matic red  glass  is  very  unusual.  In  the  region  of  i  to  2.5/1  the  metal 
which  is  used  to  color  the  glass  renders  it  more  permeable  to  heat  waves 
than  is  ordinary,  visually  transparent  glass.  The  band  at  2.9  /JL  appears 
to  be  intensified  by  the  presence  of  the  metal,  which  behaves  like  the 


100% 


80 


70 


60 
.0 

.?*> 
(0 


FIG.  39.  —  Green  glass. 


FIG.  40.  —  Ruby  glass  (a)  and  (6);  Sphalerite. 


colloidal  suspensions  just  mentioned.  In  fact,  it  is  a  pertinent  question 
whether  the  red  color  in  glasses  is  due  to  the  presence  of  metals,  such  as 
copper  and  gold,  in  the  colloidal  condition.  The  monochromatic  red 
glass  was  found  to  have  a  uniform  reflecting  power  (about  4  to  5  per  cent) 
throughout  the  spectrum  to  8  /*.  This  seems  to  show  that  the  transparent 
region  at  i  to  2  ta  is  not  due  to  the  same  cause  that  produces  an  abnormal 
transparency  on  the  short  wave-length  side  of  a  region  of  anomalous  dis- 
persion. 

BLACK  GLASS. 

(Fig.  41.     Curve  a,  £  =  2.48  mm.;  curve  6,  Schott's  Rauchglass  No.  444,  III,  ^  =  3.6  mm.) 

Curve  a  gives  the  transmission  of  a  dark  "neutral"  glass,  colored  with 
the  oxides  of  cobalt  and  nickel.  This  glass  has  a  uniform  absorption 
throughout  the  visible  spectrum.  There  are  small  absorption  bands  at 
2  and  3.5  /*,  respectively.  Curve  b  gives  the  transmission  of  a  very  dark 
glass,  which  shows  an  absorption  band  in  the  region  of  1.5  ,«. 

Considered  as  a  whole,  these  glasses  can  be  divided  into  two  groups, 


COLORED   GLASSES. 


57 


80% 


70 


•I" 

• 

§30 


20 


10 


viz,  (i)  glasses  having  absorption  bands  in  the  visible  and  in  the  infra-red 

spectrum,  and  (2)  glasses  having  a  wide  absorption  band  in  the  optical 

region,  followed  by  an  increase  in  transparency  w>th  wave-length.     The 

red  glasses  belong  to  the  latter  group,  and 

their  behavior  is  more  nearly  like  that  of 

optically  turbid  media,  the  color  being  due 

to   the  presence,  in    the   glass,   of    small 

spheres   of  metal  (see  Garnett,   loc.   cit.). 

To  the  first  group  belong  the  cobalt  blue 

and  Schott's  blue- violet  glass.     If  the  color 

in  some  glasses  is  due  to  the  metal,  one 

would  expect  great  opacity  in  the  infra-red, 

as  is  known  for  a  thin  film  of  the  metal.   If 

the  metal  is  present  in  a  colloidal  form,  the 

present    examination    of    colloidal    silver 

would  indicate  great  transparency,  with  the 

possibility  of  small  absorption  bands  in  the 

infra-red.     The  whole  question  is  in   an 

unsettled  state,  and  an  investigation  of  the 

transmission  of  various  glasses,  blown  into  FlG-  4I-~~ Black  glass- 

thin  films,  as  described  in  Carnegie  Publication  No.  65,  page  65,  seems 

highly  desirable. 

In  leaving  this  subject  it  is  of  interest  to  note  that  a  specimen  of  garnet 
(Carnegie  Publication  No.  65,  p.  59)  was  found  to  have  a  wide  absorption 
band,  with  complete  opacity  extending  from  1.2  to  2.6  //.  This  is  one  of 
the  few  substances  having  a  large  absorption  band  near  the  visible  spectrum. 

SPHALERITE  (ZnS). 
(Cleavage  piece;  ^=1.53  mm.;  transparent;  slight  yellowish  tinge;  curve  c,  fig.  40.) 

This  substance  was  previously  examined  (Carnegie  Publication  No.  65, 
p.  63),  using  for  the  purpose  a  rock-salt  prism  and  a  radiometer.  In  the 
present  examination,  the  same  mirror  spectrometer,  but  a  fluorite  prism 
and  a  bolometer  were  used.  On  account  of  the  small  dispersion  at  i  ^ 
the  rock-salt  prism  is  not  well  adapted  for  work  in  this  region.  The 
transmission  curve  of  sphalerite  appears  to  have  a  band  at  i.8/z.  On 
examination  of  the  original  data  and  curves,  it  appears  that  in  redrawing 
the  curve  for  publication  the  draftsman  placed  the  point  at  1.3  /JL  a  little 
too  high  (should  read  59  per  cent),  which  in  the  reduced  illustration  makes 
a  depression  at  1.8  /*. 

The  curve  obtained  with  the  fluorite  prism  is  given  in  fig.  40,  and 
represents  conditions  more  accurately.  A  Nernst  glower  was  used  as  a 
source,  which  gave  large  deflections  in  this  region  of  the  spectrum. 

The  transmission  curve  increases  uniformly  from  47  per  cent  at  0.6  p 
to  56  per  cent  at  i  /*,  then  decreases  uniformly  to  54  per  cent  at 


58  INFRA-RED  TRANSMISSION  SPECTRA. 

1.8  //,  beyond  which  the  transmission  drops  rapidly  to  a  minimum  at  2.9  /*, 
previously  found.  Using  this  larger  dispersion,  there  appear  to  be  no 
absorption  bands  up  to  1.9  /*.  The  study  of  zinc  sulphide  is  of  interest  in 
connection  with  luminescence  of  one  form  of  this  substance  known  as 
Sidot  blende.  In  their  study  of  the  rapidity  of  decay  of  phosphorescence 
in  Sidot  blende,  when  subjected  to  infra-red  rays,  Nichols  and  Merritt1 
found  that  the  rate  of  decay  was  the  most  rapid  when  exposed  to  infra-red 
rays  of  wave-length  0.9  p  and  of  wave-length  1.37  /*.  From  this  it  would 
seem  that  the  screen  of  Sidot  blende  possessed  broad  absorption  bands 
with  maxima  in  the  region  of  0.9^  and  1.37^.  The  luminescence  of 
Sidot  blende  appears  to  be  due  to  some  metal  dissolved  in  it.  From  the 
foregoing  experiment  on  sphalerite  (ZnS)  it  appears  that  these  absorption 
bands  are  not  due  to  the  ZnS,  but  rather  are  to  be  sought  for  in  the  dis- 
solved metal  (see  cobalt  glass),  and  in  the  cement,  in  case  a  glue  is  used  to 
secure  the  powder  in  the  form  of  a  "  screen."  The  present  study  of  colored 
glasses  gives  us  some  idea  of  what  to  expect  in  solutions  of  metals  (oxides 
of  metals?)  in  glasses;  while  the  phenomenon  maybe  further  complicated 
by  the  molecular  structure  of  ZnS,  which  in  the  form  of  Sidot  blende 
may  have  absorption  bands  not  found  in  sphalerite. 

1  E.  L.  Nichols  and  E.  Merritt:  Phys.  Rev.,  25,  p.  362,  1907. 


CHAPTER  II. 

EFFECT   OF   SPECIAL   GROUPS   OF   ATOMS   OF   RADIANT   ENERGY. 

In  the  present  chapter  it  is  purposed  to  discuss  some  of  the  relations 
found  among  the  infra-red  spectra  of  the  various  substances  examined; 
and  incidentally  to  explain  some  apparently  inconsistent  statements  made 
in  Part  I  on  the  effect  of  certain  particular  groups  of  atoms  in  producing 
characteristic  absorption  bands. 

COMPARISON  OF  ULTRA-VIOLET  AND  INFRA-RED. 

Hartley1  found  that  an  open  chain  of  CH-groups,  as  well  as  a  union  of 
C  and  N  atoms,  produced  no  absorption  bands  in  the  ultra-violet.  Neither 
could  he  identify  any  absorption  band,  of  any  of  these  substances,  with  the 
carbon  atom  in  the  benzene  ring;  the  molecules  behaved  as  though  there 
were  no  special  atoms  present.  This  is  just  the  opposite  of  what  was  found 
in  the  infra-red.  But  the  length  of  the  infra-red  spectrum  examined  is  60 
times  as  long  as  the  ultra-violet.  It  is,  therefore,  possible  to  find  relations 
in  the  infra-red  which,  on  account  of  the  narrowness  of  the  spectrum,  may 
not  be  found  in  the  ultra-violet.  In  the  infra-red  there  are  transparent 
regions  between  characteristic  absorption  bands,  which  are  often  several 
times  the  width  of  the  entire  ultra-violet  spectrum.  The  petroleum  distil- 
lates (chain  compounds)  have  a  wide  transparent  region  between  4  and  5  /*, 
just  as  Hartley  found  in  the  ultra-violet.  In  general,  if  we  were  to  examine 
a  narrow  region  of  the  spectrum,  e.g.,  the  ultra-violet,  or  at  4  /z  in  the  infra- 
red, it  appears  to  be  a  matter  of  chance  whether  or  not  absorption  bands 
will  be  found  there.  In  the  visible  spectrum  many  substances  have  ab- 
sorption bands,  i.e.,  are  colored.  If  the  eye  were  sensitive  to  rays  of 
wave-lengths  3  to  3.5^,  all  the  carbohydrates  would  appear  "colored," 
and  the  position  of  the  maximum  of  the  absorption  band  would  be  just 
as  irregular  as  those  found  in  the  visible  and  in  the  ultra-violet  spectrum. 

THE  HYDROXYL  GROUP. 

During  the  preparation  of  the  third  volume  of  Kayser's  "Handbuch 
der  Spectroscopie"  only  the  writer's  "  Preliminary  Report  on  Infra-Red 
Absorption  Spectra"2  was  available  to  the  compilers.  They  call  attention 
to  the  fact  that  "  the  writer  found  an  effect  due  to  special  groups  of  atoms, 
particularly  OH  and  CH3.  Although  many  substances,  which  contain  the 

1  Kayser:  Spectroscopy,  3,  p.  169  and  p.  170. 
2Coblentz:  Astrophysical  Journal,  20,  207,  1904. 

59 


60  INFRA-RED   TRANSMISSION   SPECTRA. 

OH-group,  have  an  absorption  band  between  2.9  and  3  //,  Coblentz  ques- 
tions whether  they  are  due  to  the  OH-group.  On  the  other  hand,  he 
thinks  the  CH3-group  causes  the  band  at  3.43/1." 

This  inconsistency  is  easily  explained.  One  need  but  examine  the 
third  volume  of  Kayser's  Spectroscopie,  pages  81,  83,  84,  88,  283,  etc., 
and  see  the  numerous  exceptions  to  "Kundt's  Law,"  to  be  convinced  that 
the  announcement  of  that  "Law"  was  premature.  Knowing  how  subse- 
quent investigators  were  misled  by  this  announcement,  it  seemed  to  the 
writer  that,  so  long  as  there  were  any  serious  exceptions,  the  effect  of 
special  groups  of  atoms  on  heat-waves  should  be  doubted  until  the  evidence 
was  much  greater  than  obtained  at  that  time.  The  mineral  brucite 
[Mg(OH2)]  caused  this  doubt;  for  if  there  is  an  effect  due  to  the  OH  group, 
one  would  expect  to  find  it  in  such  a  simple  compound  as  this  one.  The 
first  examination  of  brucite  showed  no  band  at  3  /*,  but  a  wide  band  with 
a  maximum  at  2.6  /*.  Since  then  this  mineral  has  been  re-examined,  using 
a  larger  dispersion,  and  the  large  band  was  resolved  into  its  components, 
with  maxima  at  2.5,  2.7,  and  3  /*.  In  other  words,  this  mineral  has  the 
characteristic  band  of  the  hydroxyl  group.  In  the  meantime,  numerous 
other  substances  (see  Carnegie  Publication  No.  65)  containing  OH-groups 
and  water  of  crystallization  have  been  examined.  In  Carnegie  Publication 
No.  65,  the  exceptions  to  the  rule  that  the  OH-group  has  a  characteristic 
absorption  band  at  3  /£  were  found  among  minerals  of  which  the  chemical 
constitution  is  in  doubt.  The  absorption  band  at  3  //,  found  in  substances 
containing  hydroxyl  groups,  is,  therefore,  to  be  ascribed  to  that  group  of 
atoms.  The  band  at  3  /*  found  in  the  alcohols  belongs,  therefore,  to  the 
OH-group,  and  one  ought  not  expect  (as  the  writer  stated  in  Carnegie 
Publication  No.  35,  p.  108)  to  find  a  second  band  at  6  /*  (which  is  found 
in  water),  if  the  3  /*  band  is  due  to  the  OH-group.  The  OH  group 
apparently  does  not  produce  a  harmonic  series  of  bands  such  as  are  to 
be  found  in  compounds  containing  CH2  or  CH3-groups.  Water  also  has 
a  harmonic  series  of  bands,  but  it  has  not  yet  been  shown  to  be  due  to 
the  OH-group. 

THE   EFFECT   OF   MOLECULAR  WEIGHT   OF   THE   MAXIMA. 

In  Part  I  a  search  was  made  for  a  shifting  of  the  maximum  of  an  ab- 
sorption band  with  an  increase  in  the  number  of  atoms,  or  especially  of 
groups  of  atoms,  in  the  molecule.  The  evidence  was  very  contradictory. 
In  the  case  of  the  methyl  derivatives  of  benzene,  the  maximum  of  the 
band  was  found  at  3.25  /*  in  benzene,  at  3.3  jj.  in  toluene  (C6H5CH3),  at 
3.38  //in  the  xylenes  [C6H4(CH3)2],  and  at  3.4  fi  in  mesitylene  [C6H3(CH3)3]. 
In  other  words,  by  substituting  three  CH3-groups  for  an  H  atom,  we  have 
shifted  the  maximum  from  3.25  to  3.4  /*.  Several  gases  showed  a  shift  of 
the  band,  in  the  region  of  3.2  to  3.5  //,  toward  the  long  wave-lengths  with 
an  increase  in  the  number  of  H  atoms.  On  the  other  hand,  several  nitrogen 


EFFECT   OF   MOLECULAR   WEIGHT. 


6l 


50% 


derivatives  of  benzene  showed  a  shift  of  the  maximum  toward  the  short 
wave-lengths.  An  examination  of  two  petroleum  distillates,  C8H18  and 
C^Hso,  showed  no  shift  of  the  maxima,  and  on  the,/ whole  the  evidence  of 
a  shift,  due  to  a  change  in  molecular  weight,  was  inconclusive. 

In  Carnegie  Publication  No.  65,  p.  56,  the  sulphates  showed  absorption 
and  reflection  bands  which  seemed  to  shift  toward  the  long  wave-lengths 
with  increase  in  molecular  weight.  This  data  will  be  discussed  in  the 
present  paper. 

Ever  since  the  announcement  of  "Kundt's  Law"  various  observers 
have  tried  to  find  relations  among  the  absorption  bands,  viz,  such  that 
with  an  increase  in  molecular  weight  the  band  shifts  toward  the  long 
wave-lengths.  The  results  have  just  been  discussed  in  the  first  part  of 
this  chapter.  There  is  a  physical  basis  for  expecting  a  shift  of  the  band 
with  change  in  molecular  weight.  For  example,  the  sulphate  of  the  metals 
K,  Rb,  and  Cs  have  been  compared  by  Tutton,1  who  has  shown  that  both 
as  regards  crystalline  form,  specific  gravity,  thermal  expansion,  and  corre- 
sponding refractive  indices  the  Rb  salt  lies  between  the  K  and  Cs  salts. 

In  Carnegie  Publication  No.  65,  the  sulphates  of  Mg,  Ca,  Sr,  Cd,  and 
Ba  were  found  to  have  the  maximum  of 
their  absorption  at  4.5,  4.55,  4.6,  4.6,  and 
4.63  /*,  respectively.  The  same  shifting  of 
the  maximum  was  observed  in  the  reflec- 
tion bands  of  SrSO4  and  BaSO4;  in  the 
former  the  maxima  are  at  8.2,  8.75,  and 
9.05  n,  while  in  the  latter  the  maxima  are 
shifted  to  8.35,  8.9,  and  9.1  /*. 

In  the  present  paper  all  the  available 
data  have  been  compiled,  to  show  this  shift. 
In  fig.  42  is  given  a  composite  of  the  re- 
flection curves  of  the  carbonates  [(Mg,  Ca, 
Zn,  Ba,  Pb)  CO3]  described  on  a  previous 
page.  The  maxima  are  evidently  double, 
although  it  is  not  very  apparent  in  some 
curves.  As  a  whole,  however,  the  location 
of  the  maxima  of  PbCO3  is  so  much  farther 
in  the  infra-red  than  in  MgCO3  that  there 
can  be  no  doubt  that  the  shift  is  a  real  one. 

In  fig.  43  are  given  the  graphs  of  the 
maxima  of  the  reflection  band  plotted 
against  atomic  weight  of  the  basic  atom  in  the  molecule.  The  carbon- 
ates of  Mg,  Ca,  Zn,  Sr,  Ba,  Fe,  Cu,  Pb  are  included.  In  this  figure  the 
two  maxima  of  the  complex  band  at  6.4  to  7.2  fj.  are  plotted.  When  the 


FIG.  42. —  Reflection  curves  of  carbonates. 


1  See  Miers's  Mineralogy. 


62 


INFRA-RED  REFLECTION   SPECTRA. 


maximum  is  not  sharp  and  well  defined,  several  readings  were  made  and 
plotted  in  the  figures.  This  method  of  reading  a  flat  band  establishes  a 
limit  within  which  the  maximum  may  lie.  The  two  graphs  are  approxi- 
mately parallel,  and  show  a  uniform  shift  of  the  maxima  of  Mg,  at  6.55 
and  6.86  /JL  to  6.94  fj.  and  7.18  /*,  respectively,  in  Pb.  The  carbonates  of  Fe 


200 


6.2      6.4       6.6       6.8       7.0      7.2.JUL —  11.4      /I.6     I/.8/J.-/4.0      I4.Z      14.4     74.6     I4-.8       /5/t 

FIG.  43.  —  Maxima  of  reflection  bands  of  carbonates. 

and  Cu  lie  farthest  from  the  curves;  but  they  belong  to  a  different  group 
in  Mendelejeff's  table.  The  graphs  at  n  /JL  and  14/1  are  due  to  Morse, 
see  Appendix  III. 

In  fig.  44  are  plotted  the  maxima  of  the  reflection  bands  of  the  sulphates 
[(H2,  Mg,  Ca,  Sr,  Ba)SOJ,  previously  examined  (Part  IV).    Here  there  are 


140 
I2Q 

^100 

'1 
.0  SO/ 

1  60 

40 
20 

IM 

3JC 

/ 

iLIM 

cue 

r 

Max 

IV 

/ 

Max 

/ 

-Ba 

j'Ba 

r 

Ba 

/ 

/ 

/ 

>Cd 

i 

/ 

Sir 

•/ 

Sr 

A 

•/ 

Sr 

Izn 

I    rCf 

Zn(i 

ol)t 

1 

/ 

I 

1    M 

1 

/A 

Ca 
<a 

1 

W/C 

/ 

/ 
Na 

a»K 

H* 

/ 

/ 

.2      8.3      8.4       8.5      £.6      8.7       8.8      Q.9      9.0      9.1       9.2       9.3      9.4      9.5      9.6     9.7 JU. 
FIG.  44.  —  Maxima  of  reflection  bands  of  sulphates. 


EFFECT   OF   MOLECULAR   WEIGHT.  63 

four  bands.  As  in  the  preceding,  several  readings  of  each  ill-defined 
band  are  plotted  in  order  to  get  the  limits  between  which  it  extends.  It  is 
of  interest  to  note  that  one  of  the  H2SO4  bands,  at  8.,62  p,  falls  on  the  graph 
of  the  second  maximum. 

In  fig.  45,  the  maxima  of  the  absorption  bands  of  the  sulphates  [(Mg, 
Ca,  Sr,  Cd,  Ba)  SOJ  are  plotted  (see  Part  III,  Carnegie  Publication  No. 
65,  for  data).  It  will  be  noticed  that  the  graph  of  the  band  at  4.5  p  is 
much  steeper  (the  horizontal  scale  is  different  for  these  bands)  than  are 
the  ones  farther  toward  the  infra-red.  The  maxima  of  the  absorption 
bands  lie  close  to  the  graph  drawn  through  them. 


200 


.Pb       .,.Pb 


IILMaz 


45       4&      4.7JU.    4£       4.9       5.0  6.O       6.3       6.4      6.6 

FIG.  45.  —  Maxima  of  absorption  bands  of  sulphates. 

The  second  and  third  maxima  of  the  absorption  bands  at  6.2  and  6.5  p 
are  quite  close  to  the  graph,  except  SrSO4,  which  has  but  one  maximum 
lying  midway  between  the  two  graphs.  From  this  it  would  appear  that 
the  SrSO4  band  may  be  the  mean  of  two  unresolved  bands  (see,  however, 
Part  V,  Chapter  III).  The  slant  of  these  graphs  is  less  than  in  the  pre- 
ceding one. 

For  a  discussion  of  the  graph  at  45  p  see  Appendix  III. 

Considered  as  a  whole,  the  data  presented  demonstrate  very  conclu- 
sively the  influence  of  the  molecular  weight  of  the  basic  atom  or  "ion" 
upon  the  position  of  the  maximum.  The  results  explain  why  the  increase 
in  the  number  of  the  groups  of  atoms  (see  carbohydrates)  had  no  effect 
upon  the  position  of  the  maximum.  This  is  to  be  expected  if  the  cause 
of  the  band  is  due  to  the  group  of  atoms;  but  the  position  of  the  band 


64 


INFRA-RED   REFLECTION   SPECTRA. 


depends  upon  the  metallic  (basic)  atom  to  which  the  group  of  atoms  is 
united  to  form  a  compound. 

It  is  a  remarkable  fact  that  nearly  all  the  substances  examined,  espe- 
cially the  oxides,  have  a  region  of  strong  selective  reflection  from  8  to  12  /*. 
Drude1  has  shown  that  the  optical  properties  (e.g.,  dispersion)  lead  to  the 
conclusion  that  each  atom  is  a  union  of  many  independently  vibrating 
elementary  masses  or  ions.  In  the  ultra-violet  the  absorption  band  is  due 
to  the  sympathetic  vibration  of  particles  which  have  a  charge  and  a  mass 
identical  with  the  negative  ion  or  "electron,"  while  in  the  infra-red  the 
absorption  and  reflection  bands  are  due  to  positive  ions  ("ponderable 
atoms")  which  have  a  mass  of  the  order  of  magnitude  of  the  atom.  From 
this  standpoint,  one  would  expect  to  find  a  shift  of  the  maximum  toward 
the  long  wave-lengths,  as  we  increase  the  atomic  weight  of  the  element 
which  is  attached  to  the  radical. 

The  latest  theoretical  discussion  of  this  subject  is  due  to  Einstein,2  who 
shows  how  the  theory  of  radiation,  especially  Planck's,  leads  to  a  modi- 
fication of  the  molecular-kinetic  theory  of  heat,  and  clears  up  points  here- 
tofore difficult.  He  also  shows  relations  between  the  thermal  and  optical 
properties  of  solid  bodies. 

TABLE  II. 


Element. 

Atomic  heat  = 
atomic  weight  X 
specific  heat. 

X  calculated. 

Substance. 

X  observed. 

X  calculated. 

S  andP  
Fl 

5-4 
tr 

V- 

42 

•3  •? 

CaFl  .  .  . 
NaCl 

A* 

24;  3i-6 

/* 
335    >48 

\    ..Q 

O  .    ... 

KC1 

$L.J 
6l    2 

~>  /i8 

Si  

i  8 

2O 

CaCO3 

B  

2  7 

T  r 

SiO2 

H  

2  7 

1  1 

C  

i  8 

j  j 

From  the  value  of  the  atomic  heat  of  a  substance,  he  computes  the 
maxima  of  the  bands  of  infra-red  selective  reflection.  The  observed 
reflection  bands  and  the  same  as  calculated  by  Einstein  are  given  in  table  II. 
From  this  table  it  will  be  seen  that  carbon  (diamond)  would  have  a  large 
reflection  band  at  n  //,  which  is  possible,  as  will  be  noticed  in  the  trans- 
mission curve,  fig.  30. 

This  table  shows  that  certain  calcite  maxima  are  due  independently  to 
C  at  11.4  fi  and  to  O  at  21  /*.  In  quartz  the  band  at  20  p.  is  due  to  Si  at 
20  fi  and  O  at  21  //.  From  this  line  of  reasoning,  it  would  appear  that  the 
large  reflection  band  of  carborundum  at  12.5  /*  is  due  to  C,  and  that  there 
is  another  band  at  20  JJL  due  to  Si.  A  similar  computation  for  selenium 
and  iodine  indicates  a  reflection  band  at  about  42  /*. 


1  Drude:  Ann.  der  Phys.  (4),  14,  677,  1904. 

2  Einstein:  Ann.  der  Phys.  (4),    22,  p.  181,  1907. 


CHARACTERISTICS    OF    SILICATES.  65 


CHARACTERISTIC   BANDS   OF   QUARTZ  AND   OF  SILICATES. 

About  half  the  minerals  known  are  silicates.  ^Their  percentage  com- 
position has  been  ascertained  by  quantitative  analysis,  but  little  has  been 
possible,  as  yet,  in  establishing  their  constitutional  formulae.  In  organic 
chemistry  the  constitution  of  compounds  has  been  determined  by  vapor 
density  determinations,  by  the  preparation  of  series  of  derivatives,  by  the 
replacement  of  certain  constituents  by  organic  radicals,  or  by  studying 
their  physical  properties  in  solution.  In  mineralogy  this  has,  as  yet,  not 
been  possible,  and  the  constitution  of  many  of  the  minerals  has  been 
derived  mainly  from  analogies  with  other  substances  which  are  better 
understood.  The  silicates  are  sometimes  grouped  as  follows:  disilicates 
(RSi2O5)  are  salts  of  disilicic  acid  (H2Si2O5),  and  have  an  oxygen  ratio  of 
silicon  to  bases  of  4:  i;  poly  silicates  (R2Si3O8)  are  salts  of  polysilicic  acid 
(H4Si3O8),  with  oxygen  ratio  of  3:1;  metasilicates  (RSiO3)  are  salts  of 
metasilicic  acid  (H2SiO5),  oxygen  ratio  of  2:  i;  ortho silicates  (R2SiO4),  salts 
of  orthosilicic  acid  (H4SiO4),  oxygen  ratio  of  i:  i. 

Among  the  complex  silicates,  and  often  among  the  simple  ones,  not 
only  is  the  actual  molecular  structure  in  most  cases  doubtful,  but  even  the 
simple  empirical  composition  of  many  species  is  still  unsettled.  A  single 
species  may  vary  greatly  in  composition.  This  has  been  explained  by 
regarding  the  different  forms  as  derivatives  of  a  normal  salt  in  which 
various  atoms  or  molecular  groups  may  enter.  It  is  not  surprising  then, 
in  the  present  limited  study  of  their  reflection  spectra,  to  find  no  very 
important  relations  among  the  reflection  bands  of  silicates. 

From  previous  work  it  is  difficult  to  decide  what  one  ought  to  expect 
in  the  question  of  the  coincidence  of  bands  in  SiO2  and  in  silicates.  In 
the  case  of  the  carbonates  there  are  no  bands  (except  a  small  depression 
at  4.6  IJL  in  common  with  CO)  coincident  with  those  of  CO2;  and  we  might 
expect  a  similar  condition  to  obtain  in  the  silicates  and  SiO2.  On  the 
other  hand,  in  the  carbohydrates  having  a  structure  indicating  CH2  and 
CH3  groups,  there  is  a  coincidence  of  certain  characteristic  bands  with 
those  of  ethylene  (C2H4)  and  ethane  (C2H6).  The  introduction  of  oxygen 
atoms,  however,  modifies  the  spectrum,  and,  as  a  rule,  there  is  no  longer 
a  coincidence  with  the  former  bands.  Furthermore,  in  SO2  the  maximum 
at  8.7  fi  is  in  coincidence  with  a  similar  band  found  in  many  sulphates, 
while  the  band  at  10.4  /£  is  in  common  with  a  similar  one  in  fuming  sul- 
phuric acid  and  is  probably  due  to  SO4.  From  this  it  is  difficult  to  predict 
what  one  ought  to  expect  in  the  case  of  SiO2  and  of  the  silicates.  From 
previous  observations  that  groups  of  chemically-related  substances  have 
similar  absorption  spectra,  one  would  expect  to  find  a  similar  condition  to 
obtain  in  the  various  groups  of  silicates.  The  present  investigation  is  not 
extensive  enough  to  draw  general  conclusions.  Moreover,  the  constitution 
of  the  minerals  is  in  many  cases  in  doubt,  so  that  the  lack  of  similarity  in 


66 


INFRA-RED   REFLECTION   SPECTRA. 


lljit 


I2JUL 


Rn-Silicates  » 
Wfflemite  Zn2SiO4 

i       "    . 

Spodumene  LiAl(SiOjj)2 

i     ! 

I 

(Sodium  Silicate  Na2SiO3) 

I 

Enstatite  MgSiO3 

I 

Pectolite  HNaCa2(SiO3)2 

L                      .  i    1 

Amphibole  Ca(Mg3)(SiO3)4 

—                              n 

\ 

Talc  H2Mg3(Si03)4 

1 

Serpentine  IL^MgsSiaOg 

1        i 

Deweylite      H4Mg2Ni2(SiO4)3  • 
4H20 

1        1 

Apophyllite     (H,K)Ca(Si03)2- 
H2O 

Rni-Silicates 
Topaz  (Al,F)2SiC>4 

1 

% 

Cyanite  (AlO)2SiO3 

r 

i  '  '  ; 

. 

Orthoclase  KAlSisO8 

to 

Microcline  KAlSisO8 

I         <  \ 

Albite  NaAlSisOg 

1 

i 

Muscovite  H2KAl3(SiC>4)3 

in 

Biotite  (H1K)2(Mg,Fe)2(Al,Fe)2 

(SlO4)3 

Tourmaline       (Na,Mg,Al,Fe,) 
(BOH)(Si04)s 

r 
_  i  i     i 

- 

Natrolite  Na^ljjSisOio+HjO 

i     r 

Datolite  CaB^SiC^ 

i  t 

1 

Beryl  Be3Al(Si03)a 

R^-Silicates. 
Zircon  ZrSiO4 

1            1 

'                f" 

1 

Quartz  (crystal)  SiO2 

0 

Quartz  (glass) 
7 

1,1, 

/W-                      Q^                     9fJL                   IOJU.                     lljbl                    I2/ 

Fig.  46.  —  Line  spectra  of  silicates. 

1  R11,  Valence  of  metal  is  indicated  by  the  exponent. 


CHARACTERISTIC  BANDS. 


67 


spectra  of  supposedly  related  silicates  is  not  due  to  a  fault  in  the  criterion 
for  judging  such  relations.  The  reflection  bands  of  the  various  silicates 
examined  are  represented  by  lines  in  fig.  46.  The^lines  broken  at  the  top 
indicate  that  the  reflection  bands  are  not  well  resolved.  The  silicates  of 
the  divalent  metals,  Rn-silicates,  are  metasilicates,  while  the  Rra-silicates 
are  orthosilicates,  as  noticed  on  a  previous  page.  Since  there  is  a  band  (at 
10  /*)  in  common  with  both  groups  we  are  at  liberty  to  assume  that  the 
division  into  ortho-  and  meta-silicates  is  not  a  characteristic  in  the  spectra. 
Again,  the  groups  of  chemically  related  minerals  may  or  may  not  have 
similar  spectra.  Thus,  the  pyroxenes  and  micas  have  dissimilar  spectra. 
Serpentine  and  deweylite  (and  the  feldspars)  have  similar  spectra.  Ortho- 
clase  and  microcline,  which  are  isomeric,  show  their  structure  by  their 
dissimilar  spectra.  The  amphibole  seems  out  of  place.  Spodumene,  talc, 
serpentine,  deweylite,  apophyllite,  microcline,  albite,  and  muscovite  have 
a  group  of  atoms  in  common,  which  causes  a  band  at  about  9.7  /*.  Am- 
phibole, topaz,  microcline,  albite,  tourmaline,  natrolite,  datolite,  and  beryl 
have  a  group  of  atoms  causing  a  band  at  10  p. 

Serpentine,  deweylite,  topaz,  and  albite  have  a  common  radical  causing 
a  band  at  10.5  /*.  Quartz  glass,  datolite,  orthoclase,  and  possibly  amphi- 
bole, have  a  band  in  common  at  8.8  p. 

The  group  of  triplets  in  willemite,  topaz,  and  zircon  is  unusual,  the 
maxima  being,  on  an  average,  at  10.1,  10.6,  and  n  //,  respectively.  In 
willemite  (R11)  the  bands  are  sharp  and  well  defined,  in  topaz  (R111)  the 
bands  are  not  so  well  resolved,  while  in  zircon  (R17)  the  bands  are  almost 
indistinguishable.  Whether  this  change  in  sharpness  of  bands  is  due  to 
the  variation  in  valence  is  a  pertinent  question.  Previous  work  indicates 
that  the  variation  in  the  sharpness  of  the  bands  is  due  to  the  amount  of 
oxygen  present  —  oxygen  sharpens  the  bands.  In  crystalline  quartz  and 

TABLE  III. 


Compounds  having  the 
following  groups. 

Show  characteristic  absorption  and  reflection  bands  at: 

CH2  order  CH3 

A4 
3-43 

2.96 

3-25 
7-47 
2-95 
4.78 
4-55 
3 
6-5 

A* 
6.86 

(6.1    I 
I  to    I 
I  6.15  } 

6-75 
9.08? 
(3-o) 

6.1 
8.4 
6.8 

A* 
(13-6) 

)    to    ( 

(  13-8  ) 

8.68 

6.5 
8.8 
11.4 

A* 
14 

9.8 

8.2 

9-7 
14 

A* 

u.8 

8.7 
10 
29.4 

A* 

12.95 

9.0 
10.5 

A* 

9.2 
11 

NH2  

C6H6  

NO2  

OH  

NCS 

R-SCV  . 

R-SiOx    . 

R-CCV  . 

1The  position  of  the  maxima  in  these  compounds  depends  upon  the  atomic  weight  of  the  basic  element,  R, 
with  which  the  group  or  acid  radical  is  combined. 


68  INFRA-RED  REFLECTION   SPECTRA. 

quartz  glass  there  is  a  band  in  common  at  8.4  /*,  showing  a  common  group 
of  atoms.  This  band,  with  the  five  groups  in  common  with  the  silicates, 
makes  a  total  of  six  groups  of  bands,  the  radicals  which  cause  them  being, 
as  yet,  undetermined.  These  bands  are  at  8.4,  8.8,  9.7,  TO,  10.5,  and  u  p. 
This  seems  to  substantiate  the  view  expressed  in  Carnegie  Publication 
No.  65,  p.  95,  that  in  the  silicates  there  appear  to  be  several  silicon-oxide 
radicals,  one  or  more  of  which  are  present  in  eachjnineral,  or  even  in  the 
different  specimens  of  the  same  mineral. 

In  table  III  are  given  the  characteristic  maxima  of  the  reflection  and 
absorption  bands  of  radicals  studied  in  this  and  in  previous  work. 


PART  VII. 


INFRA-RED  EMISSION  SPECTRA. 


CHAPTER  I. 

EMISSION   SPECTRA   OF   METALS   IN    HYDROGEN. 

In  Carnegie  Publication  No.  35  the  writer  presented  the  results  of  an 
examination  of  the  arc  spectra  of  various  metals.  It  was  there  shown  that 
only  the  alkali  metals  have  strong  emission  lines  in  the  infra-red.  In  the 
case  of  metals  like  copper,  iron,  and  zinc  there  was  a  weak  continuous 
spectrum  in  the  region  of  2  to  3  /*,  which  appeared  to  be  due  to  the  hot 
oxides  formed  in  the  arc.  If  this  be  true,  then  one  would  expect  to  elimi- 
nate this  radiation  by  producing  the  arc  in  an  atmosphere  of  hydrogen. 
During  the  past  year  (1907)  this  work  has  been  repeated,  using  an  arc 
inclosed  in  a  metal  case  which  was  filled  with  hydrogen.  A  Rubens 
thermopile  was  used  to  measure  the  radiation.  The  galvanometer  had  a 
full  period  of  16  seconds;  its  sensitiveness  was  i=i.jXio~10  for  a  scale  at 

1  m.    When  used  with  the  thermopile  the  temperature  sensitiveness  was 
such  that  a  deflection  of  i  mm.  =  iXio~5  °  C.  for  a  scale  at  i  m. 

The  spectrum  was  produced  by  means  of  the  rock-salt  prism  and  mir- 
ror spectrometer  (slits  0.5  mm.)  which  were  described  in  Carnegie  Publica- 
tion No.  65. 

(a)   NICKEL  ARC   IN  HYDROGEN. 

With  this  outfit  the  emission  spectrum  of  nickel  in  hydrogen  was  exam- 
ined. The  window  of  the  metal  case  inclosing 
the  arc  was  of  thin  glass  which  absorbed  all  the 
radiation  beyond  3.5  p.  The  metal  electrodes 
were  about  i  cm.  diameter,  with  the  ends  cut 
wedge-shaped.  This  produced  a  long  narrow 
arc,  an  end-on  image  of  which  was  projected 
upon  the  spectrometer  slit  by  means  of  a  1 2  cm. 
focus,  concave  mirror. 

The  spectrum  appeared  continuous  in  the 
red,  where  the  maximum  radiation  was  observed. 
The  results  are  shown  in  fig.  47,  where  a  strong 
emission  is  to  be  observed  at  0.75  //.  Beyond 

2  fj.  the  radiation  from  the  hot  electrodes  is  indicated  by  the  rapid  rise  in 
the  radiation  curve. 

The  results  show  but  little  radiation  in  the  region  of  2  /*,  where  pre- 
viously a  continuous  spectrum  was  observed.  It  is  also  to  be  observed 
that  no  strong  emission  lines  occur  at  i  p,  where  the  alkali  metals  have 
intense  lines. 

71 


FIG.  47.  —  Nickel  arc  in  hydrogen. 


INFRA-RED   EMISSION   SPECTRA. 


\ 


(b)    SPARK   SPECTRA   OF   METALS   IN   HYDROGEN. 

For  producing  a  high  potential  spark  (arc)  a  10,000- volt  transformer, 
with  100  volts,  i  to  9  amperes  in  the  primary, 
was  used.  From  one  to  three  glass  condenser 
plates,  each  having  a  capacity  of  0.0028  M.F., 
was  placed  in  parallel  with  the  electrodes.  This 
produced  a  very  intense*  arc.  An  image  of  the 
electrodes  was  projected  upon  the  spectrometer 
slit  by  means  of  a  short  focus,  concave  mirror. 
The  electrodes  were  about  3  mm.  diameter,  filed 
wedge-shaped.  They  were  mounted  in  glass 
holders  which  were  fitted  into  a  glass  vessel  of 
about  200  c.c.  capacity  by  means  of  ground 
joints  (designed  by  Dr.  Nutting),  which  per- 
mitted adjustment  of  the  electrodes  as  shown  in 
fig.  48.  The  radiation  passed  through  a  quartz 
window  about  3  mm.  thick,  hence  opaque  be- 
yond 4  //.  The  hydrogen  was  prepared  electro- 
lytically  in  an  automatic 
glass  generator  shown  in 
fig.  49. 

The  spark  (arc)  spectra 
of  the  following  metals 
were  examined :  alumi- 
num, copper,  and  calcium.  For  electrodes  of  the 
latter,  thin  pieces  were  sawed  from  a  large  bar  of 
the  metal,  which  were  then  hammered  into  wires 
about  2  to  3  mm.  in  diameter.  These  were  kept 
under  oil  to  prevent  oxidation.  The  result  of  the 
examination  was  rather  disappointing,  for  no  ap- 
preciable radiation  could  be  detected  up  to  3.5  to 
4  fit  where  the  electrodes  gave  from  3  to  10  cm. 
deflections.  This  latter  was  eliminated  by  covering 
the  spectrometer  slit  over  the  whole  length,  except  FIG.  49- —  Automatic  eiectro- 

i  i  •    i  •  -i  ,..  lytic  hydrogen  generator. 

about  2  mm.,  which  permitted  only  the  radiation 
from  the  high  potential  arc  (spark)  to  enter  the  instrument.  The  fact 
that  no  deflections  greater  than  i  mm.  were  observed  is  not  due  to  a  lack 
of  sensitiveness  of  the  instrument,  and  it  may  be  concluded  that  no  emis- 
sion lines  were  produced  which  had  an  appreciable  intensity.  This  is 
rather  surprising,  for  calcium  is  close  to  the  group  of  elements  having 
emission  lines  in  the  infra-red. 


FIG.  48.  —  Hydrogen  spark  tube. 


THE   CARBON   ARC.  73 


EMISSON    SPECTRUM   OF   THE    CARBON   ARC. 

The  emission  spectrum  of  carbon  was  previously  examined  by  the 
writer  in  connection  with  the  emission  spectra  of  metals  and  salts  of  metals 
in  the  carbon  arc.  It  was  found  that  the  violet  part  of  the  carbon  arc  had 
several  emission  lines  near  i  //,  followed  by  a  weak  continuous  spectrum 
with  a  maximum  at  about  2.5  /*.  There  seemed  to  be  a  slight  emission 
band  at  4.52  /*,  which  on  subsequent  examination  could  not  be  reproduced. 
The  writer,  therefore,  concluded  that  in  the  combustion  of  the  carbon 
electrodes  no  CO2  or  CO  is  formed,  but  that  the  carbon  passes  off  as  a 
vapor. 

Furthermore,  it  was  found  that  when  salts  of  the  alkali  metals  (Na, 
K,  Li)  were  present,  even  in  small  quantities,  there  was  a  strong  emission 
line  at  4.52  /*. 

The  radiometer  used  was  not  very  sensitive,  while  the  conclusion  that 
the  carbon  arc  was  mainly  vapor  of  carbon  did  not  seem  satisfactory, 
hence  the  work  was  repeated,  using  electrodes  of  ordinary  gas  carbon  and 
also  of  pure  Acheson  graphite.  The  latter  is  difficult  to  burn  even  when 
the  length  of  the  arc  is  only  2  mm.  long. 

In  the  present  examination  a  Rubens  thermopile  was  used.  The 
galvanometer  period  was  10  seconds,  and  its  sensitiveness  was  about 
2.5Xio~10  amperes.  The  spectrometer  slit  was  o.i  mm.  wide  and  2  mm. 
long,  and  an  image  of  the  blue  vapors  of  the  arc  was  projected  upon  it  by 
means  of  a  short  focus  mirror.  The  carbon  electrodes  were  of  the  usual 
diameter,  12  mm.,  while  two  sizes  (5  and  10  mm.  diameter)  of  graphite 
electrodes  were  used.  An  examination  was  also  made  using  one  carbon 
and  one  graphite  electrode. 

In  fig.  50  are  given  the  emission  curves  of  the  carbon  arc,  a  and  b  for 
a  solid  electrode  with  soft  core,  using  15  amperes  (D.  C.),  while  curve  c  is 
for  hollow  electrodes  (2  mm.  holes).  The  latter  was  examined  to  learn 
whether  the  air,  drawn  in  through  the  hole,  had  any  effect  —  which  appears 
negative. 

In  curves  d  and  e  are  given  the  emission  curves  observed  at  various 
times,  under  various  conditions,  when  the  hollow  carbons  contained  the 
salts  of  the  metals  NaCl  and  KC1.  The  length  of  the  arc  was  from  5  to 
7  mm.  and  in  some  instances  was  as  much  as  12  mm.  On  the  whole, 
whatever  the  current  —  from  5  to  15  amperes  —  there  is  always  an  emis- 
sion band  at  4.5  /*  when  salts  of  the  alkali  metals  are  in  the  arc,  while  the 
pure  carbon  (on  15  amperes)  had  little  if  any  selective  emission  in  this 
region. 

An  examination  was  then  made  of  the  emission  of  pure  graphite  (Ache- 
son's).  The  direct-current  arc  was  maintained  with  difficulty  and  could 
not  be  made  longer  than  4  to  5  mm. 

In  fig.  51  are  given  the  observed  results  for  the  emission  of  pure  graphite 


74 


INFRA-RED  EMISSION  SPECTRA. 


in  the  region  of  4.5  ft.    The  purity  of  this  material  may  be  judged  from 
the  entire  absence  of  the  sodium  lines;  only  the  violet  carbon  bands  were 


4 
cm 


KCl 


.    C 


/  2  3  4 

FIG.  50.  —  Emission  of  carbon  arc. 


6JJ. 


visible.     The  lower  curves  are  for  a  current  of  7  amperes  (120- volt  cir- 
cuit), while  the  two  upper  curves  are  for  a  current  of  8  amperes,  all  of 


6mm 

£    2 
.S   3 


o    / 

2mm 

i 

2mm 


3  4.  5  6// 

FIG.  51.  —  Emission  of  graphite  arc  (8  amperes). 


which  were  obtained  under  various  conditions.  On  account  of  the  diffi- 
culty in  maintaining  a  steady  arc,  no  attempt  was  made  to  map  a  strong 
emission  band  extending  from  1.2  to  1.7  /*  and  a  second  band  extending 


GRAPHITE. 


75 


from  2  to  3  /*.  The  latter  is  the  one  always  observed  and  generally 
attributed  to  oxides,  in  the  case  of  a  metallic  arc. 

It  is  to  be  observed  that  the  deflections  are  only  a  few  millimeters  as 
compared  with  the  large  deflections  when  salts  of  metals  are  present. 

An  examination  was  then  made  of  the  graphite  arc,  using  a  current  of 
4  amperes.  The  length  of  the  image  of  the  arc  projected  upon  the  spec- 
trometer slit  (which  was  also  the  length  of  the  arc  itself)  was  only  from 
2  to  3  mm.  The  arc  was  very  unsteady  and  difficult  to  keep  burning. 
The  results  are  given  in  fig.  52,  curves  a  (4  amperes)  and  d.  Curve  a  is 
for  an  arc-length  of  2  mm.,  while  d  is  for  a  length  of  about  5  mm.  For 
the  shorter  arc  the  maximum  is  shifted  toward  the  long  wave-lengths. 

In  this  same  figure,  curve  b  gives  the  emission  when  the  cold  (negative) 
electrode  was  a  gas  carbon 
(with  soft  core)  and  the  posi- 
tive electrode  was  pure  graph- 
ite. The  current  was  varied 
from  5  to  6  amperes,  and  the 
length  of  the  arc  was  from  4 
to  5  mm.  There  is  an  emis- 
sion maximum  at  4.6  //,  as 
was  found  for  the  two  graph- 
ite electrodes.  For  compari- 
son of  position  and  intensity, 
the  emission  band  of  carbon 
dioxide  in  a  Bunsen  burner 
is  given  in  curve  c,  fig.  52. 

The    reSUltS     With     the     pure  FIG.  52. -Emission  of  graphite  arc  (4  to6  amperes). 

graphite  show  that  there  is  a  sharp  emission  band  at  4.65  /*  (4.55  JJL  for  a 
longer  arc),  where  previously,  for  a  larger  current,  there  was  none.  Since 
all  the  observations  extend  over  the  same  range  of  length  of  arc,  it  appears 
that  the  occurrence  of  the  emission  band  depends  upon  the  current  and  that 
it  disappears  for  a  large  current.  This  is  certainly  extraordinary.  Since 
the  observations  on  graphite  were  made  in  quick  succession  on  the  same 
day,  as  given  here,  and  since  no  changes  were  made  in  the  apparatus,  the 
lack  of  an  emission  band  for  a  current  of  7  amperes  is  not  to  be  attributed 
to  a  fault  in  the  adjustments.  To  attribute  the  occurrence  of  emission 
bands,  when  salts  of  metals  are  present,  to  some  "catalytic"  action  caused 
by  the  presence  of  these  metals  does  not  elucidate  matters  very  much. 

Paschen  was  the  first  to  observe  that  the  emission  band  of  carbon 
dioxide  at  4.3  /*  shifts  to  the  long  wave-lengths  with  rise  in  temperature. 
At  that  time  the  dissociation  of  carbon  dioxide  at  high  temperatures  was 
but  little  investigated.  Since  the  absorption  band  of  carbon  dioxide  is  at 
4.3  /JL  and  that  of  carbon  monoxide  is  at  4.6  //,  if  the  emission  band  is  due 
to  a  pure  thermal  effect,  then  one  would  expect  to  find  that,  with  rise  in 


76  INFRA-RED   EMISSION   SPECTRA. 

temperature  and  the  consequent  dissociation  of  CO2  into  CO,  the  maxi- 
mum of  the  emission  band  of  CO2  will  shift  toward  that  of  CO.  The  writer 
has  observed  this  emission  band  of  CO2  under  various  conditions  of  tem- 
perature. In  the  emission  spectrum  of  the  Hefner  lamp  it  was  found  at 
4.36  p..  In  the  arc  spectra  of  the  salts  of  the  metals  previously  studied  it 
was  found  at  4.52  /z.  In  the  vacuum-tube  radiation  of  CO  and  CO2  the 
maximum  was  found  at  4.75  /£.  In  the  acetylene  flame  it  was  found  at 
4.4  //,  while  in  the  present  examination  it  occurs  at  4.55  to  4.65  /JL.  It  has 
always  appeared  to  the  writer  that  this  shift  of  the  emission  maximum 
with  rise  in  temperature  is  due  to  dissociation  of  CO2  into  CO,  and  no 
satisfactory  data  has  yet  been  found  to  refute  this  assumption.  The 
vacuum-tube  radiation  of  CO  and  CO2,  where  the  maximum  emission 
occurs  at  the  same  place  for  both  gases,  seemed  to  prove  the  point.  The 
present  data  furnish  further  evidence  that  the  cause  is  due  to  dissociation. 
For,  using  a  graphite  arc  on  4  amperes,  when  the  arc  is  short,  and  the 
hottest,  the  maximum  occurs  at  4.65  /*.  When  the  salts  of  the  metals  are 
introduced  into  the  arc  they  are  ionized  and  the  metals  carry  the  greater 
part  of  the  current.  The  resistance  is  reduced,  and  hence  the  tempera- 
ture (and  dissociation  of  CO2)  is  reduced.  Here  the  maximum  is  at  a 
shorter  wave-length  corresponding  to  less  dissociation  of  CO2.  While 
this  explains  certain  points  it  fails  to  account  for  the  lack  of  an  emission 
band  when  the  current  is  large,  8  to  15  amperes.  It  is  hardly  permissible 
to  assume  that  neither  CO  or  CO2  is  formed  at  this  high  temperature  and 
that  the  electrode  disappears  in  the  form  of  vapor,  though  the  writer 
made  this  assumption  at  the  conclusion  of  his  previous  examination  of 
the  carbon  arc. 

The  change  in  density  of  the  gases  in  the  two  cases  will  not  account 
for  the  observations,  for  even  the  smallest  amount  of  CO  and  CO2  is  capable 
of  emitting  perceptible  radiation,  as  was  found  in  the  experiments  with  the 
vacuum  tube.  As  a  whole,  the  mechanism  producing  this  radiation  is  not 
understood  and  further  experiments  will  be  needed  to  gain  insight  into 
this  subject. 

THE  INVESTIGATIONS  OF  MOLL. 

While  the  above  experiments  on  the  carbon  arc  were  being  concluded 
the  investigations  of  Moll1  on  infra-red  metallic  spectra  appeared  in  print. 
He  examined  the  emission  spectra  of  the  salts  of  the  metals  of  Na,  K,  Rb, 
and  Cs  in  the  carbon  arc,  using  for  the  purpose  a  rock-salt  prism,  a  ther- 
mopile, and  an  automatic  device  for  recording  the  galvanometer  deflections. 
The  apparatus  was  a  little  more  sensitive  than  the  radiometer  previously 
used  by  the  writer,  while  his  dispersion  was  almost  twice  as  great,  and 
with  the  automatic  device  he  was  able  to  explore  the  region  beyond  2  fi 
more  thoroughly  than  was  possible  by  making  personal  observations.  In 

1  Moll:  Onderzoek  van  Ultra-Roode  Spectra,  by  W.  J.  H.  Moll.  Dissertation,  Utrecht, 
1907;  Proc.  Amsterdam  Acad.,  Feb.  21,  1907. 


INVESTIGATIONS    OF   MOLL. 


77 


the  region  from  2  to  4  fi  he  succeeded  in  resolving  the  emission  into  sepa- 
rate bands,  which  the  writer,  from  the  smallness  of  the  deflections  and  the 
limited  number  of  observations,  recorded  as  a  continuous  spectrum.  He 
found  the  CO2  band  at  4.44  /JL  when  he  used  the  salts  in  the  arc,  and  also 
for  various  kinds  of  carbon  electrodes.  His  energy  curves  for  sodium  and 
for  potassium  are  given  in  figs.  53  and  54,  respectively,  in  which  the  reso- 
lution of  the  energy  in  the  region  of  1.5  to  4  jj.  into  separate  bands  is  well 
illustrated.  The  automatic  apparatus  succeeded  in  doing  what  physical 


190 

n 

100 


240 

n 


so 


70 


60 
:r> 


20 


10 


180 


1.540 


/.535 


1.530 


7.525 


/.520 


JLL  O.7S  I  1.5    2        3        4JU. 

FIG.  53.  —  Emission  of  sodium  (Moll). 

endurance  would  hardly  permit,  in  mapping  this  region.  No  curves  are 
published  for  the  carbon  electrodes,  but  considerable  comment  is  made  on 
the  fact  that  in  every  case  he  observed  an  emission  band  of  CO2,  while 
the  present  writer  did  not  find  the  band,  except  when  using  salts  in  the 
arc.  From  the  aforesaid  observations  on  the  radiation  from  graphite,  it  is 
evident  that  the  conditions  were  different  in  the  two  cases.  The  writer 
had  observed  the  same  phenomena  as  did  Moll,  who,  in  his  comments, 
missed  the  point  emphasized  by  me,  viz,  that  whether  or  not  the  spec- 
trum at  2  to  4  [JL  is  a  complex  of  small  emission  bands,  the  emission  bands 
of  the  alkali  metals  at  0.76  to  i  /*  are  the  most  intense  in  the  whole  spec- 


INFRA-RED   EMISSION   SPECTRA. 


trum,  indicating  a  much  higher  temperature  (if  the  radiation  is  purely 
thermal)  than  would  be  the  case  if  the  emission  bands  at  2  to  4  /i  were 
the  more  intense. 


620 


700 


90 


50 


60 


4-0 


30 


20 


10 


1    I 
1.540 


I    ' 
7.535 


7.530 


'    I    ' 
/.525 


7.520 


>a  0.75  7.5    2        3       4/£ 

FIG.  54.  —  Emission  of  potassium  (MoD). 

RADIATION   AT   ROOM   TEMPERATURE. 

The  distribution  of  energy  in  the  spectrum  of  a  complete  radiator  has 
been  determined  for  temperatures  varying  from  373°  to  1800°  abs.,  and 
the  constant  in  the  "displacement  law"  ^max  T=A  has  been  found  to  be 
2930  (2940,  Lummer  and  Pringsheim;  2920,  Paschen).  For  bright  plati- 
num the  value  of  this  constant  is  about  2630.  The  measurements  of 
Lummer  and  Kurlbaum1  show  that  at  low  temperatures  (492°  abs.)  the 
emissivity  of  iron  oxide  is  0.3,  while  bright  platinum  is  only  about  0.04 
that  of  a  complete  radiator  at  the  same  temperature.  The  spectral  distri- 
bution of  radiation  at  low  temperatures  has  been  given  but  little  attention. 
This  is  due  in  part  to  the  impracticability  of  reducing  the  temperature  of 
the  radiation  meter  below  that  of  the  room.  The  problem  is  therefore 
reversed,  in  that  the  radiation  meter  (bolometer,  radiometer,  or  thermopile) 


1  Lummer  &  Kurlbaum:  Verb.  Phys.  Ges.  Berlin,  17,  p.  no,  1898. 


RADIATION   FROM  RUBENS   THERMOPILE.  79 

is  allowed  to  radiate  to  what  is  generally  the  source,  which  is  at  a  much 
lower  temperature.  The  first  examination  was  made  by  Langley:1 

He  located  the  maximum  galvanometer  deflection  in  the  region  of  8  //, 
but  adds  that  the  position  of  "the  maximum  depends  upon  a  single  obser- 
vation of  some  delicacy,  which  is  liable  to  subsequent  correction."  He 
further  adds  that  for  this  temperature  difference  of  — 18°  the  maximum 
ought  to  be  at  10  to  n  «.  This  would  indicate  that  there  was  some  con- 
fusion of  ideas  as  to  what  was  really  taking  place.  The  energy  emitted 
by  the  vessel  of  ice  and  salt  is  very  small  in  comparison  to  the  amount  it 
absorbed  from  the  bolometer.  Hence  the  galvanometer  deflections  are  a 
measure  of  the  radiation  emitted  by  the  bolometer  to  the  vessel  of  snow 
and  ice,  and  the  observed  maximum  is  for  a  temperature  of  —  2°  instead 
of  —i 8°  C.  However,  since  a  bolometer  strip  is  always  at  a  higher  tem- 
perature than  the  surrounding  atmosphere,  it  is  quite  probable  that, 
although  the  observations  were  made  during  zero  weather,  the  maximum 
observed  is  really  due  to  a  higher  temperature  of  the  source  (bolometer 
strip)  than  —2°  C.  In  fact,  from  the  "displacement  law,"  which  was 
then  unknown,  and  from  our  present  knowledge  of  radiation  of  different 
surfaces,  it  is  quite  possible  that  the  temperature  of  the  bolometer  was  as 
high  as  20  to  30°  C. 

Mendenhall  and  Saunders 2  state  that  they  found  the  energy  curve  of  a 
complete  radiator  at  —  90°  C.  From  their  discussion  of  the  energy  curve 
of  a  body  at  the  temperature  of  boiling  liquid  air  where  the  emission  maxi- 
mum should  lie  at  30  /*  (which,  of  course,  is  possible  if  one  could  reduce 
the  temperature  of  the  radiometer  below  this  point  in  order  to  make 
measurements),  it  would  appear  that  they,  too,  had  not  considered  their 
radiation  curve  to  be  due  to  the  bolometer  strip.  Moreover,  the  use  of  a 
bolometer  to  deduce  a  distribution  of  energy  curve  at  low  temperatures, 
where  the  bolometer  itself  becomes  the  source,  is  not  very  satisfactory,  due  to 
the  fact  that  the  temperature  of  the  radiating  strip  can  not  be  determined. 

Lummer  and  Pringsheim3  obtained  a  small  portion  of  the  spectrum 
energy  curve  of  a  screen  at  room  temperature,  which  they  used  during 
their  radiation  experiments.  The  first  really  lucid  discussion  of  the  sub- 
ject is  due  to  Stewart,4  who  found  the  spectrum  energy  curve  of  a  Nichols 
radiometer  radiating  to  a  receiver  at  liquid-air  temperature.  The  maxi- 
mum deflections  observed  were  4  mm.  The  temperature  of  the  room 
(and  radiometer  vane)  was  24°  C.  or  297°  abs.  The  observed  maximum 

1  Langley:  Amer.  Jour.  Sci.,  31,  p.  i,  1886.     "  The  radiator  was  the  bolometer  itself  at  a 
temperature  of  —  2°  C.,  and  the  source  radiated  to  was  a  vessel  of  snow  and  salt  at  —  20°  C., 
thus  determining  the  distribution  of  energy  in  the  spectrum  of  a  surface  below  the  freezing- 
point  of  water." 

2  Mendenhall  &  Saunders:  Astrophys.  Jour.,  13,  p.  25,  1901. 

3  Lummer  &  Pringsheim:  Verb.  Phys.  Ges.,  2,  p,  163,  1900. 

4  Stewart:  Phys.  Rev.,  17,  p.  476,  1903.     (In  this  paper  figs.  4  and  6  are  interchanged.) 


8o 


INFRA-RED   EMISSION   SPECTRA. 


of  his  energy  curve,  corrected  for  slit-width,  occurs  at  9.2  //,  while  the 
value  computed  from  the  "displacement  law,"  using  the  temperature 
297°  abs.  and  A  =  2920,  would  be  found  at  9.8  /*.  If  we  compute  the 
value  of  this  constant  from  the  observed  values  of  ^max  and  T,  then 
^4  =  2722,  which  is  about  6  per  cent  lower  than  for  a  complete  radiator. 
From  experiments  on  the  radiation  from  plane  surfaces,  such  as  a  radi- 
ometer vane,  this  departure  from  the  radiation  oi  a  full  radiator  is  to  be 
expected.  Then,  too,  this  form  of  radiation  is  complicated  by  the  window 
before  the  radiometer  vane.  In  general,  the  intervening  material  between 
the  source  and  the  receiver  would  have  no  effect.  For  very  delicate  radi- 
ometers, however,  the  writer  found  (Carnegie  Publication  No.  35)  that  the 
inequality  in  the  radiation  from  the  radiometer  window  had  a  great  effect 
upon  the  position  of  the  vanes.  Just  how  much  this  would  affect  the 
vane  in  the  present  case  is  unknown.  In  the  present  investigation  a  Ru- 
bens thermopile  (20  junctions  of  iron-constantan)  was  allowed  to  radiate 
to  a  copper  vessel  at  liquid-air  temperature.  A  mirror  spectrometer  and 
rock-salt  prism,  described  in  Carnegie  Publication  No.  65,  were  employed 
to  produce  the  energy  spectrum.  The  only  changes  introduced  consisted 
K  in  replacing  the  Nernst  heater 

by  a  short  focus  mirror,  which 
projected  an  image  of  the  bot- 
tom of  the  vessel  at  liquid- 
air  temperature,  upon  the 
spectrometer  slit.  The  gal- 
vanometer used  with  the  ther- 
mopile had  a  sensitiveness  of 
i=2Xio~l°  amperes  and  a 
full  period  of  about  12  sec- 
onds. The  deflections  were 
very  unsteady,  due  to  air  cur- 
rents caused  by  the  evapo- 
ration of  liquid  air.  The 
thermopile  was  placed  in  the 
position  formerly  occupied  by 
the  radiometer  and  was  cov- 
ered by  the  inner  metal  shield, 
with  a  slit  i  by  15  mm.  area. 
The  whole  was  inclosed  in  a 
metal  tube  covered  with  felt. 
This  formed  a  more  perfect  black  body  than  is  possible  with  a  radiometer. 
The  receiver  to  which  the  thermopile  radiated  consisted  of  a  thin  cylindri- 
cal copper  vessel  about  4  cm.  diameter  and  12  cm.  long  (covered  on  the 
inside  with  copper  oxide  and  lampblack) ,  which  was  suspended  in  a  vessel 
of  liquid  air,  as  shown  in  fig.  55.  The  receiver  was  kept  stationary  and 


FIG.  55- 


THE   NERNST  GLOWER. 


8l 


the  height  of  the  vessel  of  liquid  air  was  regulated.  The  temperature 
changed  so  rapidly  that  it  was  necessary  to  begin  taking  readings  with  the 
liquid  air  just  touching  the  bottom  of  the  vessel^  when  fairly  consistent 
results  were  obtained.  The  observations  are  plotted  in  fig.  56.  Curve  a 
is  plotted  through  the  mean  values  of  the  observations,  while  curve  b  is 
drawn  through  the  highest  observed  values.  The  two  curves  are  fairly 
consistent,  considering  the  difficulties  in  making  the  observations.  In  the 
region  of  6  //  atmospheric  absorption  reduced  the  deflections. 


-4 
mm 


3 

^-  —  —  * 

6' 

2 

/' 

/      X 
•    X 

-'a'" 

'"v 

x    • 

^      X 

/ 

/ 

X    / 

^»— 

^6 

X 
X 

X 

^ 

p 

> 

5fc 

\ 

> 

x 

V 

^ 

_,„ 

X 

/ 

• 

1 

- 

•)           i            2           3           4            5           6           7           8           9           10          "           '2         I3JL 

FIG.  56.  —  Emission  spectrum  of  Rubens  thermopile. 

Curves  af  and  b'  are  the  two  lower  curves  after  correcting  for  variation 
in  slit- width,  i.e.,  reducing  them  to  a  normal  spectrum.  The  maximum 
of  these  radiation  curves  lies  at  about  9.6  //.  The  temperature  of  the 
thermopile  was  21°  C.,  or  294°  abs.  Substituting  these  values  in  the 
"displacement"  formula,  the  constant  is  ^  =  2822,  which  is  about  3  per 
cent  smaller  than  that  of  a  complete  radiator.  For  the  latter,  the  maxi- 
mum would  occur  at  about  9.95  //.  It  appears  that  the  thermopile  ap- 
proaches very  closely  to  that  of  a  Kirchhoff  radiator. 

SELECTIVE   RADIATION   FROM   THE   NERNST   GLOWER.i 

The  study  of  the  radiation  from  metal  filaments  of  incandescent  lamps 
is  6f  interest  in  connection  with  the  speculations  as  to  whether  the  great 
light  emissivity  (high  luminous  efficiency)  is  due  to  an  abnormal  emission 
in  the  visible  spectrum,  with  a  corresponding  suppression  of  the  radiation 
in  the  infra-red,  or  whether  the  effect  is  due  to  the  high  temperature  at 
which  the  lamp  is  burning.  In  the  latter  case,  the  distribution  of  energy 
in  the  spectrum  may  be  uniform  (no  discontinuities),  but  a  great  deal  of 
it  will  lie  in  the  visible  spectrum.  From  a  theoretical  consideration2  of 

1  A  detailed  description  of  this  investigation  is  given  in  the  Bulletin  of  the  Bureau  of 
Standards,  vol.  4,  p.  533,  1908. 

2Aschkinass,  Ann.  der  Phys.  (4),  17,  p.  960,  1905;  Einstein,  Ann.  der  Phys.  (4),  i7>  p* 
132,  1905;  22,  pp.  191,  569,  800,  1907. 


82  INFRA-RED  EMISSION  SPECTRA. 

the  fact,  that  the  filaments  are  metallic,  electrical  conductors  with  a  high 
reflecting  power  in  the  visible,  and  probably  reflect  uniformly  high  in  the 
infra-red,  one  would  expect  the  distribution  of  energy  in  the  spectrum  to 
follow  a  law  similar  to  that  of  platinum,  but  with  different  constants. 
The  results  thus  far  obtained  from  a  study  of  such  metals  as  tungsten 
and  osmium  support  this  hypothesis.  On  the  other  hand,  in  the  case  of 
oxides,  which  conduct  electrolytically  at  high  temperatures,  there  is  no 
data  to  form  even  a  working  hypothesis.  All  the  oxides  thus  far  examined 
have  no  strong  absorption  bands  near  the  visible  spectrum;  the  only  ex- 
ception being  the  oxides  of  the  rare  earths,  such  as  cerium,  thorium, 
lanthanum,  didymium,  erbium,  etc.,  the  compounds  of  which  have  strong, 
sharply  defined  absorption  bands  in  the  visible,  and  at  least  some  of  these 
have  absorption  bands  in  the  infra-red.  It  seems  to  be  a  characteristic  of 
the  oxides,  that  they  have  a  low  reflecting  power  (like  transparent  media, 
electrical  non-conductors)  throughout  the  infra-red  to  about  8  /j.  beyond 
which  point  they  have  strong  bands  of  metallic  reflection.  In  this  region 
of  metallic  reflection,  the  emission  will  be  suppressed *  in  proportion  to  the 
reflecting  power.  In  the  rest  of  the  spectrum,  the  emission  will  be  pro- 
portional to  the  absorbing  power  (general  absorption),  while  at  the  point 
where  there  is  a  band  of  selective  absorption  in  the  transmission  spectrum, 
there  will  be  an  emission  band  in  the  emission  spectrum,  provided  the 
radiation  is  a  purely  thermal  one,  following  Kirchhoff's  Law. 

In  the  case  of  the  Auer  mantle  the  emission  spectrum2  is  a  series  of 
emission  bands  at  i  to  2  /*,  with  practically  no  emission  in  the  region  from 
4  to  7  ^  while  beyond  9  fj.  the  spectrum  is  continuous,  and  is  apparently 
as  intense  as  that  of  a  complete  radiator  at  the  same  temperature. 

In  the  case  of  the  Nernst  glower,  which  is  a  combination  of  the  oxides 
of  cerium,  thorium,  and  zirconium,  belonging  to  the  "rare  earths,"  the 
compounds  of  which  are  noted  for  their  strong  absorption  bands,  one 
would  expect  the  emission  spectrum  to  show  strong,  sharp  emission  bands; 
at  least  one  would  hardly  expect  the  emission  to  follow  the  same  general 
law  of  spectrum  energy  distribution  as  is  known  for  metals.  This,  how- 
ever, has  been  done  in  the  past,  notably  by  Lummer  and  Pringsheim,3 
and  by  Mendenhall  and  Ingersoll.4 

The  first  two  investigators,  from  a  rather  cursory  examination  of  the 
Nernst  filament,  under  normal  power  consumption,  found  a  smooth  con- 
tinuous curve,  with  a  maximum  at  wave-length  AmsiX=i.2  p.  From  this 
and  from  the  Wien  displacement  law,  ^max  T=  const,  (const.  =  2940  for 
"black  body,"  =  2630  for  platinum),  assuming  that  the  Nernst  glower 

1  Aschkinass,  Verb.  J.  Phys.  Ges.,  17,  p.  101,  1898.     Rosenthal,  Ann.  der  Phys.    (3),  68, 
p.  791,  1899. 

2  Rubens:  Phys.  Zeit.,  6,  p.  790,  1905. 

3  Lummer  &  Pringsheim:  Verh.  Deutsch.  Phys.  Gesell.,  3,  p.  36,  1901. 

4  Mendenhall  &  Ingersoll:  Phys.  Rev.,  24,  p.  230,  1907;  25,  p.  i,  1907. 


THE  NERNST  GLOWER.  83 

belongs  to  the  same  class ,0!  radiators  as  platinum  and  a  "black  body," 
they  computed  >*max=  2450°  abs.,  and  ^min=  2200°  abs. 

Their  computed  energy  curve  of  a  "black  body/'  having  its  maximum 
emission  at  1.2 ,«  departs  considerably  from  the  observed  curve.  Men- 
denhall  and  Ingersoll  compared  the  emission  of  the  Nernst  glower  in  terms 
of  a  constant  comparison  lamp,  at  a  certain  wave-length  in  the  visible 
spectrum,  at  the  melting  points  of  gold  and  of  platinum.  From  this  they 
extrapolated  on  a  straight  line  assuming  that  Wien's  equation  holds  for 
the  glower,  and  found  the  temperature  at  normal  or  any  desired  power 
consumption.  This  leads  to  erroneous  values  (which  are  probably  too 
high,  as  will  be  shown  presently),  due  to  the  fact  that  the  spectrum  is  the 
composite  of  numerous  emission  bands,  which  rapidly  increase  in  intensity, 
in  the  short  wave-lengths,  with  rise  in  temperature.  They  found  the 
normal  temperature  to  be  2300°  abs.,  disagreeing  with  a  recent  determina- 
tion by  Hartman,1  who,  by  means  of  thermocouples  of  different  thickness 
placed  against  the  glower,  correcting  for  heat  conduction  by  extrapolating 
to  a  temperature  corresponding  to  an  infinitely  thin  couple,  found  the 
temperature  to  be  1800  abs.  Although  this  method  had  previously  been 
extensively  used,  with  fair  success,  in  measuring  temperature  of  gas- 
flames,  it  is  not  suited  to  the  glower,  in  which  there  is  no  layer  of  hot  gas 
to  even  partially  compensate  for  the  heat  lost  by  conduction. 

That  the  Nernst  glower  emits  selectively  in  the  visible  spectrum,  has 
been  shown  by  Kurlbaum  and  Schulze,2  who  found  a  minimum  of  emission 
at  0.52  //,  which  became  fainter  with  rise  in  temperature,  and  disappeared 
entirely  at  high  temperatures. 

To  this  brief  review  of  what  has  been  done  on  the  Nernst  glower  may 
be  added  a  paper  by  its  inventor,3  who  showed  that  the  conductivity  is 
electrolytic,  while  Kauf mann 4  showed  that  in  spite  of  the  entirely  different 
inner  mechanism  of  conduction  of  a  gas  in  a  vacuum-tube,  and  in  a  Nernst 
glower,  the  electrodynamic  phenomena  are  nevertheless  very  similar. 

The  present  investigation  of  the  Nernst  glower  consists  in  mapping 
the  distribution  of  energy  in  the  spectrum,  varying  the  power  consumption, 
and  hence  the  temperature. 

The  apparatus  used  in  this  work  consisted  of  the  mirror  spectrometer,5 
used  in  the  preceding  experiments,  a  perfectly  clear  fluorite  prism,  having 
an  angle  of  60°  and  circular  faces  33  mm.  in  diameter,  and  a  bolometer 6 
with  a  hemispherical  reflecting  mirror.  The  bolometer  strip  and  spectrom- 
eter slit  were  0.6  mm.  wide,  or  about  4'  of  arc.  The  upper  part  of  the 

1  Hartman:  Phys.  Rev.,  17,  p.  65,  1903. 

2  Kurlbaum  &  Schulze:  Verb.  Phys.  Gesell.,  5,  p.  428,  1903. 

3  Nernst:  Zeit.  fur  Electrochemie.  6,  p.  41,  1899. 

4  Kaufmann:  Ann.  der  Phys.  (4),  2,  p.  158,  1900;  5,  p.  757,  1901. 

5  For  adjustments  see   "Investigations  of  Infra-red  Spectra,"  Carnegie  Institution  of 
Washington  Publication  No.  35,  1905. 

6  Described  in  this  volume,  Appendix  II. 


84  INFRA-RED   EMISSION   SPECTRA. 

spectrometer,  containing  the  collimating  mirrors,  and  the  Wadsworth 
mirror-prism  outfit,  were  entirely  inclosed  by  a  thin  sheet-metal  box,  lined 
with  black  velvet.  The  spectrometer  slit  was  covered  with  a  clear  plate 
of  fluorite.  The  openings  in  the  top,  for  adjusting  the  mirrors,  were  closed 
with  soft  wax,  while  the  hole  admitting  the  axis  for  rotating  the  mirror- 
prism  table  was  tightened  with  a  nut  and  packing. 

Within  the  box,  and  below  the  level  of  the  mirrors,  were  placed  vessels 
containing  phosphorous  pentoxide  and  sticks  of  potassium  hydroxide, 
which  entirely  eliminated  the  absorption  bands  of  CO2,  and  water-vapor 
from  the  emission  curves.  A  water-cooled  shutter  was  placed  before  the 
spectrometer  slit,  and  the  Nernst  glower,  inclosed  in  an  asbestos  case  to 
prevent  air-currents,  was  placed  close  to  the  shutter.  Observations  were 
also  made  without  inclosing  the  glower,  to  prove  that  the  effects  observed 
are  not  due  to  stray  radiation  from  the  asbestos  case.  In  spite  of  all  that 
has  been  written  about  bolometers,  it  may  be  added  that  there  was  no 
"  drift,"  unless  a  poor  storage  battery  was  accidentally  used. 

The  auxiliary  galvanometer  of  5.3  ohms  resistance,  with  a  single  swing 
of  4  to  5  seconds,  had  a  current  sensibility  of  i=i.6  to  i.5Xio~10  ampere. 
A  greater  sensitiveness  for  the  same  period  would  have  been  possible,  by 
using  a  lighter  suspension.  The  present  suspension  of  ten  needles  was 
just  heavy  enough  so  as  not  to  be  affected  by  tremors. 

Using  a  battery  current  of  0.04  ampere  the  computed  temperature 
sensibility  was  5°Xio~5  C.,  which  was  generally  far  in  excess  of  that  re- 
quired. The  deflections  were  reduced  to  14  to  15  cm.,  by  inserting  resist- 
ance in  series  with  the  galvanometer.  The  individual  readings  would 
vary  by  i  mm.  or  less  than  i  per  cent,  which  is  as  close  as  the  nature  of 
the  work  required,  since  the  actual  deflections  were  as  high  as  2000  cm. 
Furthermore,  at  high  temperatures  (especially  when  operated  above  nor- 
mal power  consumption)  the  filament  would  deteriorate  in  emission  by 
that  amount  during  the  series  of  observations. 

The  calibration  curve  of  the  fluorite  prism  was  constructed  from  the 
refractive  indices,  found  by  Paschen,1  which,  after  plotting  all  the  obser- 
vations made  by  different  observers,  seems  to  be  as  close  to  the  most 
probable  values  as  observations  will  permit.  Unfortunately  the  dispersion 
curve  passes  through  a  double  curvature  at  1.5  /*,  just  where  the  energy 
spectra  have  their  maximum.  In  the  region  of  1.5  ft,  the  correction  for 
purity,  so-called  " slit-width"  correction,  is  a  maximum. 

The  wave-lengths  in  the  calibration  curve  were  plotted  to  the  fourth 
decimal  place,  so  that  there  is  a  certainty  of  the  values  to  at  least  the 
second  decimal  place.  This,  however,  is  of  less  importance  than  the 
value  of  the  slit-width  correction,  which  was  made  according  to  Paschen,2 
the  values  being  obtained  from  a  curve  plotted  on  a  large  scale  to  insure 

1  Paschen:  Ann.  der  Phys.  (4),  4,  p.  299,  1901. 
3  Paschen:  Ann.  der  Phys.  (3),  60,  p.  714,  1897. 


THE  NERNST  GLOWER.  85 

an  accuracy  greater  than  required  in  the  work.  In  a  few  cases  a  correction 
was  made  for  the  reflecting  power  of  the  silver  mirrors,  but  it  was  found 
negligible  except  in  the  visible  spectrum. 

With  this  apparatus,  a  series  of  energy  curves' was  obtained,  varying 
the  power-consumption  from  16  watts  (the  lowest  at  which  the  glower 
would  conduct  on  no  volts)  to  123  watts,  which  is  far  above  the  normal. 
The  energy  curves,  which  were  continuous,  underwent  great  variations 
in  appearance  with  rise  in  temperature.  At  2.5  and  3.5  /*  depressions 
would  generally  appear  in  the  curves,  which  could  not  be  attributed  to 
experimental  errors,  and  since  previous  work  by  others  seemed  to  show 
that  the  spectrum  is  continuous,  an  attempt  was  made  to  locate  the  disa- 
greement in  the  apparatus  (in  the  calibration,  or  in  the  slit-width  correc- 
tion curve) ,  but  nothing  would  account  for  it  until  the  filament  was  run  on 
a  2ooo-volt  transformer  which  permitted  the  heating  of  the  glower  at  a  very 
low  power-consumption.  At  the  lowest  temperature  the  glower  was  a 
grayish  red.  The  results  are  given  in  fig.  57  (no- volt  A.  C.  glower  No. 
1 1 8),  and  are  entirely  different  from  anything  hitherto  recorded  in  the 
emission  of  solids  in  the  infra-red.  At  the  lowest  temperatures  (800°  to 
900°  C.)  the  bands  in  the  region  of  long  wave-lengths  are  the  most  in- 
tense. As  the  temperature  increases,  the  bands  in  the  region  of  2.5  /£  in- 
crease very  rapidly  in  intensity,  so  that  by  the  time  the  temperature  has 
increased  to  1000  to  1100°  the  intensity  of  the  group  of  bands  at  2  /*  is 
far  in  excess  of  those  at  5.5  /*. 

The  depression  at  3  to  3.5  /t  is  to  be  noticed,  for  it  persists  even  at 
normal  power-consumption.  The  region  at  2.5  ft  is  also  to  be  noticed,  for 
the  curves  at  higher  temperatures  often  show  a  slight  depression,  not  attrib- 
utable to  experimental  errors.  As  the  temperature  rises  (curve  e,  fig.  57) 
new  emission  bands  appear,  notably  at  2.5  and  4  /*.  This  shift  of  the  maxi- 
mum of  intensity  of  the  bands,  with  increase  in  temperature,  is  to  be  ex- 
pected, if  the  emission  is  a  purely  thermal  one,  following  Kirchhoff's  law, 
and  is  the  most  conspicuous  illustration  yet  recorded. 

In  fig.  58  are  given  the  emission  curves  for  the  glower  (200  volts,  serial 
No.  1 1 8)  at  19.6  and  102.5  watts,  respectively.  It  is  to  be  noticed  that  the 
emission  curve  has  already  become  smooth  and  continuous,  with  but  two 
maxima,  at  1.4  and  5.5  /*,  respectively.  In  this  figure  curve  b  is  one-tenth 
the  scale  of  curve  a. 

On  the  whole,  from  whatever  point  of  view  we  consider  the  data  at  hand, 
it  is  evident  that  even  after  the  emission  spectrum  has  become  apparently 
continuous  it  does  not  follow  so  simple  a  law  as  has  been  established  for 
solids  emitting  continuous  spectra.  It  is  also  evident  that  any  estimation 
of  the  temperature  of  the  glower  based  on  these  laws  will  lead  to  erroneous 
results.  From  a  commercial  point  of  view  the  efficiency  of  such  a  radiator, 
in  which  the  emissivity  is  abnormally  high  at  0.6  to  0.7  /£,  while  the  maxi- 
mum at  1.2  p  is  abnormally  low,  must  be  much  higher  than  that  of  some 


86 


INFRA-RED  EMISSION  SPECTRA. 


FIG. 


57.  —  Emission  spectrum  of  Nernst  glower,    a  =  2,  b  =  3, 
e=  7.1,  and/=  10.6  watts,  respectively. 


9/1 
c=  4.2,  d=  6.2, 


THE  NERNST  GLOWER.  87 

substance  having  a  radiation  law  similar  to  platinum,  in  which  case,  in 
order  to  attain  a  similar  intensity  in  the  visible,  the  maximum  at  1.2/1 
rises  to  extremely  high  values. 

The  results  obtained  are  for  filaments  made-^from  different  lots  of 
material,  the  2  20- volt  glower  being  at  least  5  years  old.  The  observa- 
tions have  been  made  at  different  dates,  on  different  filaments,  all  in  dupli- 
cate, some  quadruplicate,  and  there  is  reason  for  feeling  that  what  has 


FIG.  58.  —  Emission  spectrum  of  Nernst  glower;  (a)=  19.6  watts,  (b)=  102.5 
watts;  Xmax  =  I-4SA1  and  1.32/01,  respectively. 

been  observed  is  a  reality.  No  doubt  on  examination  of  other  makes  of 
filaments,  there  will  be  found  a  variation  in  the  finer  details  of  the  emission 
bands,  at  low  temperatures,  but  the  gross  results  can  hardly  be  modified 
without  employing  a  much  larger  dispersion  and  a  much  narrower  bolom- 
eter. Some  of  these  bands  coincide  with  certain  ones  found  in  the  emission 
spectrum  of  the  Nernst  "heater,"  the  results  of  which  are  given  in  Carnegie 
Publication  No.  35. 


88  INFRA-RED  EMISSION  SPECTRA. 

RADIATION   FROM    METAL   FILAMENTS. 

The  measurement  of  very  high  temperatures  is  based  upon  an  extra- 
polation of  the  laws  governing  the  energy  emitted  by  a  body  with  change 
in  temperature.  Our  knowledge  of  these  laws  is  confined  to  the  radiation 
from  platinum  and  from  a  uniformly  heated  cavity  (so-called  black  body) 
which  is  the  nearest  approach  to  a  complete  radiator.  The  remarkable 
progress  made  in  the  development  of  processes  requiring  an  accurate 
knowledge  of  the  temperatures  involved  makes  it  imperative  to  study  the 
laws  of  radiation  of  various  substances  with  variation  in  temperature. 
The  object  of  this  and  of  subsequent  investigations  is  to  gain  an  insight 
into  these  laws.  In  order  to  determine  these  radiation  laws  it  is  generally 
necessary  to  study  the  spectrum  energy  curves,  using  for  the  purpose  a 
prism  that  is  transparent  to  heat  rays,  and  some  sort  of  very  sensitive  heat- 
measuring  device,  such  as,  for  example,  a  bolometer  or  a  thermopile.  It 
is  also  possible  to  study  the  total  radiation  emitted.  The  chief  difficulty 
in  studying  these  so-called  radiation  constants  of  substances  lies  in  the 
impossibility  of  determining  the  temperature  of  the  radiating  surface. 

The  curves  showing  the  distribution  of  energy  in  the  normal  spectrum 
of  all  solid  bodies  thus  far  studied  are  unsymmetrical  with  respect  to  the 
maximum,  having  the  appearance  of  the  probability  function,  modified  by 
suitable  constants.  The  solids  heretofore  studied,  e.g.,  platinum,  in  which 
it  was  possible  to  determine  the  approximate  temperature,  have  spectrum 
energy  curves,  which  are  represented  fairly  well  by  the  function, 

(i)  £  =  c1X~ae-c^r 

In  the  case  of  a  complete  radiator,1  or  so-called  "  black  body,"  the 
exponent  a=  5,  while  for  platinum,  a  =6. 

In  order  to  determine  the  constants  of  the  above  equation  from  the 
spectrum  energy  curves,  it  is  necessary  to  know  the  temperature  of  the 
radiator.  Fortunately  the  index  a  may  also  be  obtained  from  the  spectrum 
energy  curve  in  which  the  temperature  T  is  constant  (E=  galvanometer 
deflections,  ^= wave-length,  are  variable),  without  knowing  the  actual 
temperature,  for  it  can  be  shown  from  equation  (i)  that  the  ratio  of  the 
emissivities  (the  observed  bolometer-galvanometer  deflections)  for  any 
two  wave-lengths,  X  and  ^max,  is: 


from  which  the  value  of  a  may  be  obtained.     It  was  found  by  Paschen 

1  Paschen:  Ann.  der  Phys.  (3),  vol.  58,  p.  455;  vol.  60,  p.  663,  1897;  (4),  vol.  4,  p.  277, 
1901.     Lummer  &  Pringsheim:  Verh.  deutsche  phys.  Gesell.,  i,  p.  215,  1899. 


METAL  FILAMENTS. 


89 


that  for  carbon,  platinum,  etc.,  the  value  of  a  was  in  agreement  with  that 
obtained  from  a  knowledge  of  the  temperature  of  the  radiator. 

With  this  equation  it  is  possible  to  obtain  somp  idea  of  the  probable 
total  emissivity  of  a  radiating  body,  as  to  whether  it  is  proportional  to  the 
4th  power  (0—1  =  4  f°r  a  black  body,  0—1  =  5  f°r  platinum),  or  to  some 
higher  power  of  the  absolute  temperature.  Of  course  the  assumption  is 
made  that  the  emissivity  function  is  similar  to  that  of  platinum  and  of  a 
black  body.  The  appearance  of  the  energy  curves  for  various  tempera- 
tures will  give  some  clue  as  to  the  admissibility  of  this  assumption,  which 
is  nothing  more  than  has  been  made  by  previous  observers.  How  far  this 
assumption  falls  short  of  the  observed  facts,  may  be  seen  in  figs.  57  and  58 
for  the  Nernst  glower,  which  has  a  spectrum  composed  of  numerous 
emission  bands.  With  substances  whose  energy  spectra  undergo  no  change 
in  contour  with  change  in  temperature,  it  does  not  seem  unreasonable  to 
apply  our  knowledge  gained  from  the  behavior  of  platinum  under  similar 
conditions,  especially  since  the  filaments  are  metals,  electrical  conductors, 
which,  theoretically,1  should  have  similar  emissive  properties.  That  the 


o  ^o  40  eo  so  100 

FIG.  59.  —  Radiation  constant  (a)  of  Nernst  glower. 

method  is  open  to  criticism  is  admitted,  but  until  a  better  one  is  suggested, 
the  present  method  is  the  only  one  available  without  a  knowledge  of  the 
temperature  of  the  radiator.  The  apparatus  used  in  this  work  consisted 
of  a  mirror  spectrometer,  a  fluorite  prism,  and  a  bolometer,  mentioned  in 
the  description  of  the  results  on  the  Nernst  glower. 

The  variation  of  the  radiation  constant  a  for  the  Nernst  glower  with 
rise  in  temperature  is  shown  in  fig.  59,  from  which  it  will  be  seen  that  it 
decreases  from  a  value  of  7  at  1 8  watts  to  5.3  at  80  to  120  watts.  These 
values  are  taken  from  the  smooth  energy  curves,  such  as  those  shown  in 

1  Aschkinass:  Ann.  der  Phys.  (4),  17,  p.  960,  1905.  Einstein:  Ann.  der  Phys.  (4),  17,  p. 
132,  1905;  22,  pp.  191,  569,  800,  1907. 


90 


INFRA-RED   EMISSION   SPECTRA. 


fig.  58.     The  curve  for  102  watts  was  irregular  in  outline  and  the  values 

52 1 1 1 r— — i 1 1     of  a  obtained  at  different  wave-lengths 

(indicated  by  crosses,  circles,  dots, 
squares,  etc.)  undergo  great  variations. 
On  the  whole,  it  is  evident  that  it  is 
hardly  permissible  to  obtain  this  con- 
stant on  the  assumption  that  the  ra- 
diation law  of  the  Nernst  glower  is 
similar  to  that  of  platinum  or  of  a 
complete  radiator.  The  energy  curve 
of  a  complete  radiator  having  a  maxi- 
mum emission  at  1.45  //  falls  far  be- 
low the  Nernst  glower  curve  (a,  fig. 
58)  at  5.5  JM,  which  shows  that  the 
emission  at  1.45  //  is  not  as  intense  as 
it  should  be  if  the  emissivity  were 
similar  to  a  complete  radiator.  For 
the  same  reason,  the  spectral  energy 
curve  of  a  complete  radiator  having 
a  maximum  at  1.32  p.  lies  far  above 
the  glower  curve  (6,  fig.  58)  at  5.5  ft. 
In  fig.  60  is  shown  a  series  of 
energy  curves  of  an  incandescent  fila- 
ment (3  cm.  long)  of  osmium,  when 
on  an  energy  consumption  of  curve 
0=2.44  and  curve  6=7.38  watts,  re- 
spectively. The  corresponding  tem- 
peratures, observed  with  an  optical  pyrometer,  using  red  absorption  glass, 
are  1607°  and  2000°  K.66/l  C.,  respectively,  while  the  computed  tempera- 
tures, on  the  assumption  that  the  radiation  constants  are  the  same  as  for 
platinum  (for  A  =1.35  and  1.2  /*),  are  1670°  C.  and  1907°  C.,  respectively. 


2345 
FIG.  60.  —  Radiation  from  osmium. 


Radiation  Constant 

Oi  0*  vl$ 

a 

Tungi 

ten 

•2O 

s 

. 

• 

& 

« 

X 

m 

© 

1234-56  7  8    WattS 

FIG.  61.  —  Radiation  constant  (a)  of  osmium. 


These  filaments  were  in  glass  bulbs  similar  to  that  of  an  incandescent 
lamp,  and  had  short  side  tubes  with  fluorite  windows.  Fig.  61  shows  the 
value  of  a  of  osmium  (of  fig.  60)  for  variation  in  energy  consumption. 


METAL  FILAMENTS.  9! 

Except  for  the  lowest  temperatures  the  value  of  a  obtained  by  this  method 
is  about  6.4.  The  surface  became  roughened,  which  lowers  the  value  of 
this  "constant."  The  weakness  of  this  method  lies  in  the  difficulty  of  ob- 
taining the  values  of  the  emissivities,  particularly  the  maximum  of  the 
energy  curve.  Since  the  filaments  are  so  narrow  that  the  prism-face  is 
only  partly  covered,  the  height  of  the  ordinates  will  depend  upon  the  cone 
of  energy  falling  upon  the  prism  face.  For  this  reason  the  energy  curves 
of  the  different  substances  can  not  be  compared.  The  value  of  a  for  a  short 
tungsten  filament  (see  fig.  61)  was  found  to  be  7.3  for  one  energy  curve. 
Further  experiments  on  another  filament  gave  a  value  of  a  =6. 88  for  the 
mean  of  18  computations  and  5  energy  curves.  The  values  of  a  for  an 
untreated  carbon  filament,  for  various  values  of  energy  consumption,  are 
plotted  in  fig.  62.  As  with  osmium,  the  temperature  was  raised  from  a 


/  2          3          4  5          6  7          8          9  Watts 

FIG.  62.  —  Radiation  constant  (a)  of  untreated  carbon  filament. 

low  red  to  the  normal  condition.  Apparently  the  emissivity  constant,  a, 
drops  from  6.4  at  about  900°  to  5.3  at  1800°  to  2000°  C.  Paschen  found 
no  such  variation  for  the  specimens  of  carbon  examined.  Neither  did  he 
find  a  variation  in  a  for  the  oxides  of  copper  and  of  iron.  On  the  other 
hand,  Lummer  and  Kurlbaum,  by  measuring  the  total  radiation  emitted, 
found  that  with  rise  in  temperature  the  emissivity  of  these  oxides  approached 
that  of  a  complete  radiator.  This  difference  may,  of  course,  be  possible, 
just  as  in  the  present  examination  of  the  Nernst  glower  the  value  of  which 
was  found  to  vary,  while  Mendenhall  and  Ingersoll,1  by  using  a  total 
radiation  method  of  comparison  with  platinum,  could  not  establish  with 
certainty  any  variation  of  a  with  temperature.  An  inspection  of  figs.  58 
and  59  shows  that  this  may  be  possible,  especially  in  the  latter  where  the 
energy  curve  appears  to  have  two  maxima.  For  the  "Helion"  filament, 
which  is  a  carbon  filament  upon  which  is  a  deposit  of  silica,  the  constant 
0=8.3  when  new,  and  decreases  to  6.3  after  aging.  For  tantalum  the 
value  of  a  is  6.5.  From  the  energy  curve  of  the  acetylene  flame  published 
by  Stewart  (Phys.  Rev.  16,  p.  123)  the  value  of  a  is  about  n.  A  plati- 
num strip,  50  X  1.5  X  0.02  mm.  inclosed  in  a  glass  bulb  with  a  fluorite 
window,  was  also  examined.  Estimating  temperatures  from  the  position 

1  Mendenhall  &  Ingersoll:  Phys.  Rev.,  25,  p.  i,  1907. 


92  INFRA-RED  EMISSION  SPECTRA. 

of  the  ^max  0*max  T  =  2620)  it  was  found  that  the  "  constant "  a  decreased 
from  a  =  8.5  at  900°  C.  to  a  =  6.3  at  1100°  C. 

Using  metal  filament  lamps  (commercial  no- volt)  under  "normal" 
working  conditions,  for  metallized  carbon  a  =  6.1,  for  tantalum  a  =  6.3,  for 
tungsten  a  =  6.6,  and  for  osmium  a  =  6.9.  In  all  cases  the  value  of  a 
was  found  to  decrease  with  rise  in  temperature. 

Theoretically,  this  variation  in  a  must  occur  at  some  stage  in  the  tem- 
perature (either  an  abrupt  decrease  at  some  fixed*  temperature,  or  a  uni- 
form decrease  throughout  the  range),  otherwise  a  temperature  would  be 
attainable  at  which  the  total  radiation  is  greater  than  that  of  a  complete 
radiator  at  the  same  temperature. 

In  the  "  black  body  "  the  reflection  is  zero.  The  Nernst  glower,  as 
well  as  the  oxides  in  general,  have  a  very  low  reflecting  power;  hence,  if 
the  radiating  layer  is  of  sufficient  thickness,  the  emissivity  of  the  oxides 
must  be  almost  as  great  as  that  of  a  "  black  body,"  as  has  just  been  found 
for  the  Nernst  glower,  at  high  temperatures.  Some  oxides,  having  absorp- 
tion bands  in  the  visible  spectrum,  e.  g.,  CeO2,  when  in  thin  layers  (Wels- 
bach  mantle)  and  at  high  temperatures  will  have  a  higher  luminous  effi- 
ciency than  the  same  material  in  thick  rods,  e.  g.,  the  Nernst  glower. 
In  the  next  chapter  it  will  be  shown  that  the  greater  the  electrical  con- 
ductivity the  more  continuous  will  be  the  emission  spectrum  of  the  oxides. 
Furthermore,  mixtures  of  oxides  like  mixtures  of  gases  in  a  vacuum  tube 
have  composite  emission  spectra.  This  indicates  a  possible  method  of 
analysis  of  mineral  solutions,  and  it  is  hoped  to  make  a  further  examina- 
tion into  this  question. 


CHAPTER  II. 

SELECTIVE    RADIATION    FROM   VARIOUS    SOLIDS. 
INTRODUCTION. 

Our  knowledge  of  the  emission  of  radiant  energy  from  various  sub- 
stances with  change  in  temperature  is  extremely  limited,  being  confined 
to  platinum  in  the  case  of  metallic  electrical  conductors,  to  several  gases 
in  vacuum  tubes,  to  water-vapor  and  carbon  dioxide  in  the  Bunsen  flame, 
to  carbon,  to  the  oxides  of  copper  and  iron,  and  to  the  radiation  from  a 
uniformly  heated  cavity  or  complete  radiator.  The  emission  spectra  of  the 
Bunsen  flame  and  of  gases  in  vacuum  tubes  were  found  to  be  composed 
of  sharp  emission  bands  superposed  upon  a  weak  continuous  spectrum. 
The  solids  were  found  to  have  smooth  continuous  emission  spectra,  and  it 
seems  to  be  the  general  expectation  to  find  (see  Kayser's  Spectroscopie, 
vol.  2,  pp.  135  and  284,  also  Rudorf,  Jahrb.  Radioakt.  und  Elektronik,  4, 
385,  1908)  that  all  solids  emit  continuous  spectra. 

To  Paschen  is  due  the  credit  for  the  first  systematic  study  of  the  spectral 
distribution  of  radiant  energy  from  various  solids,  and  from  the  Bunsen 
flame.  Subsequent  work  by  others  has  been  but  little  more  than  the 
establishment  of  the  so-called  radiation  constants  to  a  greater  number  of 
significant  figures  than  was  possible  by  Paschen,  with  the  facilities  at  his 
disposal.  Great  credit  is  due  to  Lummer  and  Pringsheim  for  establishing 
the  limits  within  which  the  radiation  laws,  notably  Wien's  law,  hold.  It 
must  be  said,  however,  that  Paschen  had  these  limits  partly  established; 
but  he  insisted  that  the  discrepancies  between  theory  and  experiment  were 
due  to  errors  of  observation.  In  this  brief  summary  it  is  not  possible  to 
present  the  subject  fully,  but  after  working  over  the  data,  one  can  not  help 
feeling  that  it  is  extremely  unfortunate  that  the  results  of  these  investiga- 
tions are  beclouded  with  controversies  as  to  whom  belonged  the  credit  for 
doing  or  suggesting  this,  that,  or  some  other  thing  in  connection  with  the 
work. 

The  best  proof  of  KirchhofFs  law  of  the  proportionality  of  emission  and 
absorption  is  due  to  Paschen,1  who  found  that  the  intensity  of  the  emission 
of  the  CO2  band  at  4.4  /£,  when  using  a  column  of  gas  7  cm.  long,  was  as 
great  as  for  a  column  of  gas  33  cm.  long.  In  other  words,  the  intensity 
of  the  radiation  was  as  great  as  that  of  a  complete  radiator  for  the  same 

1  Paschen:  Ann.  der  Phys.  (3),  53,  p.  26,  1894. 

93 


94  INFRA-RED   EMISSION   SPECTRA. 

wave-length  and  at  the  same  temperature.  Unfortunately  the  number  of 
radiating  substances  of  which  it  is  possible  to  determine  even  an  approxi- 
mate temperature  is  extremely  small.  Hence,  the  work  presented  on  the 
following  pages  can  not  be  more  than  a  qualitative  proof  of  KirchhofFs 
law  of  proportionality  between  emission  and  absorption.  It  has  numerous 
applications,  however,  particularly  in  studying  substances  having  sharp 
emission  (hence  sharp  absorption)  bands.  Numerous  substances  are  given 
here,  of  which  it  has  not  been  practicable  to  study  the  absorption  spectra. 
Their  emission  spectra  give  us  some  idea  of  the  nature  of  their  absorption, 
the  only  difference  being  that  the  emission  bands  are  more  intense,  due  to 
the  greater  thickness  (and  the  higher  temperature)  of  the  substance  exam- 
ined. Another  feature  of  the  results  obtained,  which  is  new,  is  the  extreme 
sharpness  of  the  emission  bands.  Moreover,  the  maxima  of  the  emission 
and  absorption  bands  coincide,  although  the  temperatures  at  which  the 
two  sets  of  observations  were  made  differ  by  800  to  1000°  C.  The  posi- 
tions of  the  sharp,  well-defined  maxima  are  not  affected  by  change  in 
temperature.  This  is  in  marked  contrast  with  the  results  of  Konigsberger1 
for  the  limited  region  of  the  visible  spectrum,  in  which  he  found  that  for 
certain  selectively  absorbing  substances  the  maximum  of  the  absorption 
band  shifts  toward  the  long  wave-lengths  with  rise  in  temperature,  and 
with  the  results  of  Paschen  on  the  emission  bands  of  CO2  and  water-vapor, 
which  shifted  with  rise  in  temperature,  some  toward  the  long,  others  toward 
the  short  wave-lengths.  The  data  may  also  prove  to  be  of  use  in  deter- 
mining whether  pleochroism  is  an  inter-  or  intra-molecular  phenomenon. 
For  example,  the  absorption  spectrum  of  adularia  shows  a  band  at  3.2/1, 
which  in  the  emission  spectrum  is  shifted  to  2.9  /£.  The  latter  band  is 
characteristic  of  silicates,  whether  in  the  emission  or  absorption  spectrum. 
The  present  work  may  be  considered  an  examination  of  the  emission  spectra 
of  electrical  insulators,  or  transparent  media.  The  only  previous  work 
done  on  this  subject  is  due  to  Rubens,2  who  examined  the  radiation  from 
the  Auer  mantle,  as  well  as  mantles  composed  of  the  pure  oxides  of  cerium 
and  of  thorium.  The  difficulty  experienced  by  him  was  the  elimination  of 
the  emission  spectrum  of  the  hot  gases  which  was  superposed  upon  that 
of  the  oxides  composing  the  mantle.  Hence,  if  he  had  examined  a  mantle 
of  zirconium  oxide  he  would  not  have  been  able  to  detect  an  emission 
band  which  occurs  at  4.3  /*. 

The  substances  used  in  the  present  investigation  were  in  the  form  of 
solid  rods  made  in  an  oxy-hydrogen  flame  or  in  the  form  of  thick  layers 
of  the  substance  placed  upon  a  heater.  The  rods  were  heated  by  an  electric 
current  from  the  secondary  of  a  2000-volt  (300-watt)  transformer.  These 
rods  had,  of  course,  to  be  heated  initially  with  an  alcohol  or  blast  lamp 
until  they  became  conducting,  just  as  is  necessary  with  the  Nernst  glower. 

1  Konigsberger:  Ann.  der  Phys.  (4),  4,  p.  796,  1901. 

2  Rubens:  Phys.  Zs.,  6,  p.  790,  1905. 


VARIOUS   SOLIDS.  95 

The  resistance  placed  in  the  primary  circuit  of  the  transformer  to  regulate 
the  current,  and  the  low  capacity  of  the  transformer,  acted  as  a  "ballast" 
to  the  radiating  substances.  The  rods  were  provided  with  platinum  ter- 
minals, sealed  in  the  ends,  and  were  securely  mounted  in  incandescent 
lamp  sockets.  After  heating  them  until  they  became  conducting,  they 
were  mounted  securely  before  the  spectrometer  slit.  The  substances  that 
could  not  be  melted  and  formed  into  rods  were  made  into  a  paste  and 
spread  upon  the  "heater  tube"  of  a  Nernst  lamp.  The  "heater  tube" 
consisted  of  a  hollow  porcelain  tube  about  5  cm.  long  and  8  mm.  diameter 
covered  with  a  coil  of  fine  platinum  wire,  and  was  used  in  preference  to  a 
platinum  strip  on  account  of  its  rigidity  and  ease  in  handling.  It  gave 
the  same  results  as  the  same  material  on  a  platinum  strip.  The  spectrom- 
eter, the  fluorite  prism,  and  the  bolometer  were  previously  used  in  ex- 
amining the  Nernst  glower.  It  is  important  to  notice,  however,  that  by 
inclosing  the  optical  parts  of  the  instrument  it  was  possible  to  entirely 
eliminate  the  atmospheric  absorption  bands,  which  had  not  been  done 
successfully  by  previous  experimenters.  The  emission  curves  are,  there- 
fore, within  experimental  errors,  an  exact  portrayal  of  the  distribution  of 
the  energy  emitted  in  different  parts  of  the  spectrum.  Unfortunately  it  is 
not  possible  to  determine  the  temperature  of  the  radiating  body,  the  thick- 
ness of  which  would  have  to  be  specified,  in  many  cases.  A  thermocouple 
can  not  be  applied  on  account  of  the  loss  of  heat  by  conduction.  It  is,  of 
course,  absurd  to  attempt  to  measure  temperatures  with  an  optical  py- 
rometer. For  example,  the  rod  of  oligoclase  was  a  perfectly  transparent 
glass,  and  emitted  no  light  other  than  that  due  to  sparking  of  the  platinum 
terminals,  although  at  a  temperature  of  noo  to  1200°  C.  Nevertheless,  a 
substance  such  as  iron  oxide  would  have  emitted  light  when  at  the  same 
temperature,  while  both  emit  strongly  in  the  infra-red.  A  further  example 
is  the  rod  of  topaz,  which  was  an  opaque  white  mass.  On  withdrawing  it 
from  the  oxy-hydrogen  flame,  it  was  accidentally  stroked  with  the  iron 
forceps,  when  the  parts  so  stroked  emitted  a  dull  red  light,  due  to  the 
greater  emissivity  of  the  iron  oxide,  while  the  untouched  parts  retained 
their  usual  white  color. 

Instead  of  temperatures,  the  energy  consumption  is  given;  also  the 
dimensions  of  the  rods,  the  lengths  being  the  distances  between  the  platinum 
terminals.  The  diameters  are  in  some  cases  not  very  accurate,  owing  to 
the  irregularity  of  some  of  the  filaments.  The  voltage  was  measured  with 
an  electrostatic  voltmeter,  joined  to  the  terminals  of  the  rod.  The  current 
was  measured  with  a  milliammeter,  of  a  suitable  range  to  insure  accuracy. 

It  is  manifestly  impossible  to  have  the  smoothness  of  the  surface  and 
the  thickness  of  the  radiating  layer  the  same  for  all  substances.  The 
distance  (about  2  cm.)  of  the  radiator  from  the  slit  and  the  bolometer 
sensibility  will  also  vary.  No  attempt  has  therefore  been  made  to  obtain 
the  emission  curves  of  the  various  substances  at  the  same  temperature, 


90  INFRA-RED   EMISSION   SPECTRA. 

and  to  plot  them  to  the  same  scale.  The  purpose  of  the  present  examina- 
tion is  to  map  the  distribution  of  energy  in  the  various  spectra,  hoping  that 
at  some  future  time  it  may  be  possible  to  make  a  more  extended  study  of 
certain  substances  which  show  unusual  emission  spectra. 

RADIATION  FROM  ELECTRICALLY  HEATED  SOLIDS. 
Prominent  among  this  group  are  a  series  of  silicates,  which  have  an 
emission  band  in  common  at  2.88  /JL  (characteristic  of  SiO2)  which  is  as 


Z          3         4          5          6 

FIG.  63.  —  Emission  spectrum  of  albite. 


sharp  as  any  yet  found  in  gases.  The  absorption  spectra  of  many  of  these 
compounds  are  recorded  in  Part  III  of  these  investigations  (Carnegie 
Publication  No.  65). 

In  order  to  give  the  reader  some  idea  of  the  conditions  under  which 


SILICATES. 


97 


the  data  was  obtained,  a  rough  estimate  is  made  of  the  temperature  at 
which  a  complete  radiator  would  emit  light  of  a  color  similar  to  that  given 
out  by  the  substance  under  investigation.  The  fact  that  soot  from  the  gas 
flame  was  burned  off  the  rods  not  emitting  light  would  indicate  that  the 
temperature  was  above  600°  C.,  and  in  all  cases  the  estimated  temperature 
is  greater  than  800°  C. 

The  length  of  the  rods  used  depended  upon  the  melting-point.  For 
example,  the  "soft  glass "  rod  was  very  short  on  account  of  its  softening 
at  the  temperature  necessary  to  keep  it  electrically  conducting.  The  ends 
of  the  rods  were  shielded  from  the  spectrometer  slit.  All  these  curves  are 
reduced  to  the  normal  spectrum  by  dividing  the  observed  galvanometer 
deflections  by  the  slit-width  expressed  in  wave-lengths.  In  this  work  the 
unsteadiness  of  the  bolometer  prevented  an  accurate  mapping  of  small 
emission  bands  occurring  beyond  6  //. 

ALBITE  (NaAlSi3O8). 

(Rod  8  mm.  long,  tapering  from  1.8  to  2.8  mm.  in  diameter.  Energy  supplied,  7.1  and  8.2 
watts.  Curves  a  and  b,  fig.  63.  Temperature  800°.  Transmission  given  in  Carnegie 
Publication  No.  65,  p.  65.) 

The  rod  was  translucent  and  seemed  to  emit  no  light  except  that  re- 
flected from  the  platinum  terminals.  The  emission  band  at  2.88  /JL  is 


40 


30 


10 


r 


\ 


01  2345678/2 

FIG.  64.  —  Emission  spectrum  of  orthoclase  (var.  adularia). 

more  intense  than  the  one  found  in  orthoclase,  just  as  was  found  in  the 
absorption  spectrum.  Two  other  bands  are  noticeable  at  4.1  and  4.5  /*, 
respectively. 


98 


INFRA-RED   EMISSION   SPECTRA. 


ORTHOCLASE  (van  ADULARIA)  [KAlSiaOs]. 

(Rod  8  mm.  long,  2  mm.  diameter.     Energy  supplied,  6.2  and  8.9  watts.     Curves  a  and  &, 
respectively,  of  fig.  64.    Transmission,  Carnegie  Publication  No.  65,  p.  64.) 

This  substance  emitted  a  little  more  light  than  albite,  although  it  was 
more  transparent.    The  emission  band  at  2  /*  is  prominent.     The  one  at 


/  23456  7 

FIG.  65.  —  Orthoclase  (a);  alumina  and  silica  (6);  alumina  and  feldspar. 

2.88  fi  is  shifted  from  its  position  at  3.2/1  in  the  absorption  spectrum, 
from  which  it  would  appear  that  the  group  of  atoms  causing  the  absorp- 
tion is  different  in  the  two  cases.  An  examination  of  the  transmission 
spectrum  using  polarized  light  will  be  necessary  to  determine  the  true 


SILICATES. 


99 


position  of  the  absorption  band.    The  band  at  4.1  and  4.5  fj.  are  in  common 
with  the  other  feldspars. 


ORTHOCLASE  (FELDSPAR) 

(Rod  7  by  2.2  mm.     Energy,  7.5  watts.     Curve  a,  fig.  65.    Transmission,  Carnegie  Publica- 

tion No.  65,  p.  64.) 

This  sample  was  translucent,  due  probably  to  air-bubbles.     Its  color 
on  7.5  watts  was  slightly  red,  due  in  part  to  reflected  light  from  the  elec- 


3*56 
FIG.  66.  —  Oligoclase. 


trodes.  The  emission  bands  at  2,  2.88,  4.1,  and  4.5  ft  are  in  common  with 
the  preceding,  with  the  possibility  of  a  small  band  at  3.1  /*,  to  be  noticed 
in  fig.  66. 


IOO 


INFRA-RED  EMISSION  SPECTRA. 


OLIGOCLASE 


f  3  NaAlSi308  1 
1  CaAl2Si308    j 

(Rod  9  by  2.8  mm.     Energy  supplied.     Curve  a =8.6,  6  =  13.3,  and  £  =  29.4  watts. 
Curves  a  and  b,  fig.  66.    Transmission,  Carnegie  Publication  No.  65,  p.  64.) 

The  original  crystal,  as  well  as  the  glass  rod,  were  perfectly  transparent. 
The  rod  showed  no  color  on  suddenly  throwing  off  the  current.  The 
platinum  terminals  melted  on  16  watts,  but  the  rod  showed  no  color.  As 
in  the  preceding  feldspars  there  are  bands  at  2,  f. 88,  3.1,  4.1,  4.5,  and  7  //. 
The  absorption  band  at  4.8  /z  seems  to  be  shifted  to  4.5  /*  in  the  emission 
spectrum.  Curve  c  shows  the  distribution  of  energy  when  the  rod  (another 
sample,  plotted  to  a  different  scale)  was  heated  until  viscous,  see  fig.  91. 


2345 
FIG.  67.  —  Amphibole. 


BfJL 


AMPHIBOLE  (TREMOLITE?)  [CaMg3(SiO3)4]. 

(Rod  6  by  1.5  mm.     Energy  supplied,  2  ( ?)  and  4.8  watts.     Curves  a  and  &,  fig.  67. 
Transmission,  Carnegie  Publication  No.  65,  p.  64.) 

At  the  highest  energy  consumption  the  color  of  the  filament  was  a  light 
yellow,  corresponding  to  a  temperature  of  1200  to  1300°.    The  emission 


SILICATES. 


IOI 


curve  is  quite  different  from  the  preceding,  due  in  part  to  the  higher  tem- 
perature. The  absorption  band  at  6  JJL  appears  as  a  depression  in  the 
emission  spectrum. 

WOLLASTONITE  (CaSlO3).          > 

(Rod  10  mm.  long,  tapering  from  3  to  4  mm.  diameter.     Energy  supplied,  18  watts. 
Curves  a  and  &,  fig.  68.) 

This  rod  was  made  from  a  pure  transparent  glass,  supplied  by  Dr. 
Allen  of  the  Geophysical  Laboratory  of  the  Carnegie  Institution  of  Wash- 
ington, which  became  a  white  crystalline  mass  on  melting  in  the  oxy- 
hydrogen  flame.  It  was  rendered  conducting  with  difficulty,  and  could 
not  be  heated  uniformly. 


/         2        3        4        5 

FIG.  68.  —  Wollastonite. 


The  silicate  band  at  2.9  /*  is  prominent,  while  a  new  band  appears  at 
3.7  //.  Curve  b  gives  the  emission  from  the  coolest  side  of  the  rod,  while 
curve  a  represents  the  emission  from  the  hottest  side.  The  latter  is  an 
excellent  illustration  of  the  rapid  increase  in  intensity  with  temperature, 


102 


INFRA-RED  EMISSION  SPECTRA. 


of  the  emission  bands  on  the  side  of  the  spectrum  toward  the  short  wave- 
lengths. Further  examples  will  be  found  in  figs.  77  and  78.  In  this  and 
in  the  preceding  substance  the  emission  bands  are  not  so  sharp  as  usual, 
which  may  be  due  to  the  greater  molecular  weight  of  the  base. 

PORCELAIN  (PYROMETER  TUBING). 
(Hollow  rod  15  by  2  mm.  (hole  i  mm.).     Energy  supplied,  9.2  watts.     Fig.  69.) 

The  sample  of  pyrometer  tubing  examined  was  heated  to  a  light-red 
color  (1400°)  to  keep  it  conducting.     The  energy  spectrum  is  marked  for 


o        /        a        3       •»        5        6        7        a       s/^ 
FIG.  69.  —  Porcelain. 

its  strong  emission,  with  sharp  maxima  at  1.8,  2.1,  2.83,  3.7,  4.1,  and 
4.5  ft,  and  with  indications  of  bands  beyond  6  /*.  Porcelain  is  made  from 
a  hydrous  aluminum  silicate. 

GLASS  ("SOFT  GLASS"). 

(Rod  3  by  2  mm.,  heated  to  dull  red  color.     Curve  a,  fig.  70.     Transmission, 
Carnegie  Publication  No.  65,  p.  65.) 

The  substance  examined  was  a  piece  of  ordinary  "soft"  white  glass 
tubing,  drawn  into  a  solid  rod.  There  are  emission  bands  at  2,  2.86,  3.6, 

4.4,  and  5.5  /£. 

MAGNESIA  (PYROMETER  TUBING). 

(Hollow  rod,  12  by  1.5  mm.  (hole  i  mm.).     Energy  supplied,  6.6  watts.     Curve  6,  fig.  70.) 

This  material  is  used  to  insulate  thermo-couples,  and  conducts  only  at 
high  temperatures.  It  is  probably  a  mixture  of  magnesium  oxide  (see 
fig.  80)  and  silica.  The  temperature  was  probably  1200°  to  1400°.  The 


COBALT   GLASS. 


I03 


emission  spectrum  is  conspicuous  for  two  regions  of  strong  emissivity  at 
1.6  and  5  /*,  respectively,  with  a  deep  depression  at  3.5  /*.  The  emission 
bands  at  1.6,  2.7,  5,  5.5,  and  7.1  /*  are  not  well  resolved,  but  their  presence 
can  not  be  doubted. 


a        3        4        5        6 

FIG.  70.  —  Glass  (a);  Magnesia. 


9JLL 


GLASS  (COBALT  BLUE). 
(Rod  12  by  2  mm.     Energy  supplied,  5.4  and  7.5  watts.    Curves  a  and  6,  fig.  71.) 

This  rod  was  heated  to  a  dull  red.    The  emission  spectrum  is  quite 
different  from  that  of  soft  glass,  but  there  are  not  such  prominent  bands 


10 

14 
12 

O>     p 

CO     o 

"E 

UJ 

6 
4 
2 

F 

>^ 

A 

\ 

(2 

/ 

v\ 

y^*~* 

V 

t 

J 

v 

I 

/ 

2 

V 

£ 

\ 

// 
^? 

234-5 
FIG.  71.  —  Cobalt  glass. 


8/6 


104 


INFRA-RED   EMISSION   SPECTRA. 


as  one  might  expect  from  a  knowledge  of  the  absorption  bands  at  1.6  and 
2.2  fi  to  be  noticed  in  fig.  37.  The  silicate  band  at  2.87  /*  is  prominent. 
Other  bands  occur  at  2,  3.6,  4.6,  and  5.5  JJL,  respectively. 

SPODUMENE  [LiAl(SiO3)2]. 

(From  Pennington  County,  South  Dakota.     Irregular  rod,  12  by  1.5  mm.     Energy 
supplied,  5.7  and  9.5  watts.     Curves  a  and  &,  fig.  72.) 

This  mineral  contains  cerium,  lanthanum,  and»other  "rare  earths"  as 
impurities.  It  can  be  melted  in  a  blast  flame.  Considerable  light  was 
emitted,  due  to  the  presence  of  impurities.  The  emission  spectrum  shows 
the  silicate  band,  sharply  defined  at  2.88  /*,  with  other  bands  at  2,  4.1,  4.5, 
4.9,  and  6.5  j«. 


0  I  £345 

FIG.  72.  —  Spodumene. 

BERYL  [Be3Al2(SiO3)6]. 
(Rod  10  by  3  mm.     Energy,  7  to  8  watts.     Curve  a,  fig.  73.) 

The  rod  was  an  opalescent  milky  glass,  although  the  original  crystal 
was  a  transparent  green.  The  temperature  was  probably  1100°.  The 
emission  spectrum  is  unusual  in  appearance,  with  a  sharp  maximum 
superposed  upon  it  at  2.8  /*.  Other  ill-defined  maxima  appear  to  be  at 
1.7,  2.4,  2.9,  3.6,  4.4,  and  4.8 /£. 


TOPAZ.  105 

RUTILE  (TiO2). 

(Flat  plate  8  mm.  long,  tapering  from  1.5  to  1.8  mm.  wide  and  0.25  mm.  thick.     Energy 
supplied,  6  watts.     Curve  b,  fig.  73.     Transmission,  Carnegie  Publication  No.  65,  p.  67.) 

This  mineral  was  heated  to  a  bright  red  color  corresponding  to  a 
temperature  of  perhaps  1000°.  The  emission  spectrum  shows  maxima  at 
2.4,  3.2,  5,5,  and  7.0  JJL.  The  transmission  spectrum  is  too  low  to  show 
these  as  absorption  bands;  but  the  band  at  3.1  ft  is  visible  in  the  trans- 
mission spectrum  of  brookite  (TiO2) .  This  substance  is  a  good  conductor 
of  electricity  at  this  temperature,  but  a  very  poor  radiator  of  light  rays. 


/  £345678          9/1 

FIG.  73.  —  Beryl  (a);  Rutile. 

TOPAZ  [(AlF)2SiO4]. 

(Rod  13  mm.  long,  2  to  2.5  mm.  diameter.     Energy  supplied,  0.05  and  0.07  ampere,  — 
probably  15  to  21  watts.     Curves  a  and  &,  fig.  74.) 

This  rod  was  built  up  from  the  massive  transparent  mineral,  and 
rendered  conducting  by  the  vapor  set  free  from  the  albite  cement  used  to 
secure  the  platinum  terminals  to  the  rod.  The  vapor  left  a  narrow  con- 


io6 


INFRA-RED  EMISSION  SPECTRA. 


ducting  streak  along  one  side  of  the  rod,  which  heated  the  topaz.  The 
curve  is  conspicuous  for  the  sharpness  of  its  emission  bands,  which  occur 
at  1.4,  2.85,  4.1,  4.5,  and  7.5  //. 


SILICATES. 


107 


APATITE  [Ca5F(PO<)3]. 

(Rod  12  by  1.5  mm.     Energy  supplied,  9.3  and  n.8  watts.     Curves  a  and  &,  fig.  75. 
Transmission,  Carnegie  Publication  No.  65,  p.  58.) 

The  emission  spectrum  shows  emission  bands  at  2.9  and  4.5  //.  The 
rod  was  made  from  a  dark  massive  specimen  which  may  have  contained 
silica,  whence  the  band  at  2.9  /*  in  both  the  emission  and  absorption  spec- 


40 


30 


£20 


10 


}j 


/ 


A 

A 


\ 


/  2  3  4  5 

FIG.  75.  —  Apatite. 


8/1 


trum.  On  the  other  hand,  many  oxides,  known  to  be  free  from  silica, 
have  a  strong  emission  band  in  this  region,  from  which  it  would  appear 
that  this  band  is  characteristic  of  the  oxides. 

ALUMINUM  OXIDE  (A12O3). 

(Rod  14  mm.  long,  tapering  from  2.5  to  3  mm.  in  diameter.     Energy  supplied,  0.025  an(^ 
0.115  ampere.     Curves  a  and  6,  fig.  76.) 

The  rod  was  built  up  from  the  pure  powder  in  an  oxy-hydrogen  flame. 
The  emission  curve  is  conspicuous  for  the  great  variation  in  intensity  of 
its  emission  bands  which  occur  at  1.4,  2,  3.1,  4.7,  5.2,  and  7.5 (?)  /*.  The 
emission  curve  differs  but  little  from  the  one  of  topaz  except  that  the  latter 
has  the  silicate  band  at  2.85  /*.  In  fig.  65,  curve  c  shows  the  radiation 
from  a  rod  made  from  alumina  and  i  per  cent  of  feldspar,  and  curve  b, 
fig.  65,  gives  the  radiation  from  a  rod  made  of  alumina  and  i  per  cent  of 
silica.  In  both  cases  the  silica  band  at  2.87  /i  is  superposed  upon  the 
spectrum  of  the  aluminum  oxide. 


io8 


INFRA-RED   EMISSION   SPECTRA. 


ZIRCONIUM  OXIDE  (ZrO2). 

(Rod  5  by  1.4  mm.     Energy  consumption,  3.5  (900°),  4.8,  5.8,  7.5  watts,  curves 
a,  b,  c,  d,  fig.  77;  9.6,  12.1,  and  14.5  (1400°)  watts,  curves  a,  b,  c,  fig.  78.) 

The  specimen  examined  was  a  fragment  from  a  furnace  wall.     It 
probably  contained  some  "binder,"  although  from  the  curve  for  the  pure 


3456 

FIG.  76.  —  Aluminum  oxide. 


material  (fig.  79)  the  foreign  substance  contributed  but  little  to  the  emission 
bands.  The  fine  platinum  terminals  were  wound  around  the  ends  of  the 
rod  which  was  about  10  mm.  long  (5  mm.  between  the  terminals). 

The  curves  are  conspicuous  for  their  sharp  emission  band  at  4.3  /*, 


ZIRCONIUM   OXIDE. 


I09 


which  remains  superposed  upon  the  continuous  background  even  at  a 
bright  yellow  heat,  corresponding  to  a  temperature  of  about  1400°. 


23456 
FIG.  77.  —  Zirconium  oxide. 


This  series  of  energy  curves  is  one  of  the  best  illustrations  yet  recorded 
of  the  gradual  shift  of  the  maximum  of  intensity  of  emission  toward  the 
short  wave-lengths  with  rise  in  temperature.  On  an  energy  consumption 


no 


INFRA-RED  EMISSION  SPECTRA. 


of  3.5  watts,  the  maximum  of  the  envelope  of  these  emission  bands  lies  at 
4  to  5  fi}  and  shifts  steadily  to  the  short  wave-lengths,  being  at  about  1.8  ju 
for  an  energy  consumption  of  14.5  watts.  The  pure  oxide  is  not  an  efficient 
radiator  of  white  light,  and  only  becomes  so  when  a  small  amount  of  cerium 
or  thorium  oxide  is  added,  which  combination  is  the  Nernst  glower  already 


70 


234567 
FIG.  78.  —  Zirconium  oxide. 


mentioned.  Aside  from  the  sharp  emission  lines  at  2.8  and  4.35  /*  there 
are  wide  hazy  bands  at  2  and  2.4^  (appears  on  7.5  watts),  while  from 
5  to  6  ft  there  is  a  wide  band  which  is  evidently  unresolved,  the  maximum 
shifting  toward  the  short  wave-lengths  with  rise  in  temperature.  The 
extraordinary  rapidity  which  characterizes  the  growth  of  the  emissivity  at 


MAGNESIUM  OXIDE.  Ill 

1.5  to  2  £  is  worthy  of  notice.  In  one  case  where  the  ratio  of  emissivity 
at  4.3  fj.  for  a  given  increase  in  energy  is  50,  it  is  almost  200  at  2  /*.  In 
fig.  77  the  curves  are  all  for  practically  the  samg  sensibility,  71,  of  the 
instrument.  In  fig.  78  the  sensibility  of  the  instrument  was  60.  These 
units  are  arbitrary,  being  the  galvanometer  deflections  obtained  by  un- 
balancing the  bolometer  by  a  constant  amount.  For  this  substance  the 
radiating  rod  was  short  and  the  voltage  too  low  to  be  measured  with  the 
electro-static  voltmeter.  The  ordinary  voltmeter  used  affected  the  tem- 
perature of  the  radiator  slightly,  so  that  the  energy  consumption  recorded 
is  not  so  accurate  as  in  the  other  observations. 

EMISSION  SPECTRA  OF  SOLIDS  ON  NERNST  "HEATER  TUBE." 

In  examining  the  radiation  from  solids  placed  upon  a  heater  it  is  pos- 
sible to  have  some  of  the  radiation  from  the  latter  transmitted  through 
the  former.  If  the  substance  to  be  examined  is  in  the  form  of  a  fine  powder 
and  the  layer  is  from  0.5  to  1.5  mm.  thick  there  is  but  little  chance  for 
the  radiation  to  be  transmitted  from  the  heater.  This  fact  enables  one  to 
study  the  selective  emission  of  solids  which  can  not  be  formed  into  solid 
rods,  and  is  applied  in  studying  the  following  list  of  substance. 

ZIRCONIUM  OXIDE  (ZrO2). 

(Layer  of  oxide  0.8  to  1.5  mm.  thick,  on  Nernst  heater  tube  from  which  the  clay 
covering  had  been  removed.     Fig.  79,  curves  o,  &,  and  c.) 

The  zirconium  oxide  was  heated  to  a  dull  red,  which  was  somewhat 
lower  than  for  the  rod  heated  electrically  (curve  a,  fig.  77).  Curves  a  and 
c  give  the  emission  of  different  samples  of  the  commercial  product,  the 
electrical  properties  of  the  latter  being  very  bad  when  used  for  the  body 
of  a  glower.  Curve  b  shows  the  emission  of  a  sample  which  has  good 
electrical  properties.  It  contained  less  than  o.oi  per  cent  of  silica,  and 
was  the  purest  obtainable  sample.  From  this  it  is  evident  that  the  sharp 
maxima  are  not  due  to  silica.  The  emission  spectra  are  very  similar  and 
are  conspicuous  for  two  sharp  emission  bands,  with  maxima  at  2.78/1 
(curve  0,  maximum  at  2.83  p)  and  at  4.3  /*,  respectively.  Smaller  maxima 
appear  at  2,  2.4,  3.2,  4.7,  and  5.4  p.  From  a  comparison  of  these  curves 
with  those  given  in  figs.  77  and  78  it  appears  that  in  the  latter  sample  the 
"binder"  used,  probably  yttria,  has  but  little  influence  upon  the  sharp 
emission  bands. 

MAGNESIUM  OXIDE  (MgO). 
(Curve  c,  fig.  80.) 

The  magnesium  oxide  (curve  c,  fig.  80)  spectrum  shows  two  wide 
bands  at  3  and  5.3  /*,  respectively,  with  a  smaller  band  at  2.1  p.  Some- 
what similar  conditions  were  observed  in  fig.  70. 

In  fig.  80,  curve  a  gives  the  emission  spectrum  of  the  material  used  in 
a  Nernst  glower  placed  upon  a  heater-tube.  The  general  outline,  and 


112 


INFRA-RED   EMISSION   SPECTRA. 


especially  the  emission  bands,  are  to  be  observed  in  the  radiation  curves 
of  the  glower,  given  in  fig.  57.  In  curve  b,  fig.  80,  is  shown  the  emission 
spectrum  of  a  commercial  "  heater-tube."  The  covering  is  some  refractory 
substance,  perhaps  containing  aluminum  oxide  and  a  silicate  binder. 
There  is  a  sharp  band  at  2.85  n  and  hazy  bands  at  4.7,  5.3,  and  6.3  /*. 


2345 

FIG.  79.  —  Zirconium  oxide. 


7JUL 


URANIUM  OXIDE  (U2O3);  CERIUM  OXIDE  (CeO2);  THORIUM  OXIDE  (ThO2). 

(Curve  o=U2O3;  &=ThO2;  c=CeO2;  fig.  81.) 

The  uranium  oxide  is  a  greenish-brown  powder  which  gives  a  smooth 
continuous  spectrum  with  hazy  maxima  at  2.8  and  3.4  /*,  respectively. 

The  thorium  oxide,  curve  b,  shows  no  marked  emission  bands,  and  the 
whole  spectrum  is  suppressed  to  7  /*.     Beyond  this  point  the  emissivity  is 


CERIUM   AND   THORIUM   OXIDES.  113 

high,  corresponding  closely  to  that  of  a  complete  radiator,  as  was  shown 
by  Rubens,  using  a  gas-mantle  of  this  material. 

The  cerium-oxide  curve  shows  a  strong  emissjon  at  2  to  3  //,  as  was 
found  by  Rubens,  with  possible  bands  at  4.4  and  7.5  //.  It  was  shown  by 
Rubens  that  beyond  7  fj.  the  emissivity  approaches  that  of  a  complete 
radiator. 

23 


0/2345.  678  $jU 

FIG.  80. — Nernst  glower  (a);  Nernst  "heater-tube"  (6);  Magnesium  oxide. 

In  the  Auer  gas  mantle  (90  per  cent  ThO2  +  i  per  cent  CeO2)  the  cerium 
oxide  acts  as  a  sensitizer  does  on  a  photographic  plate,  by  making  an  ab- 
sorption band  in  one  region  of  the  spectrum  without  affecting  the  rest. 

The  energy  curve  of  an  electrically  heated  filament  of  the  same  com- 
position as  the  Auer  mantle  is  given  in  the  Bulletin  of  the  Bureau  of 
Standards,  vol.  5,  No.  2,  1908.  The  energy  curve  is  entirely  different 


INFRA-RED  EMISSION  SPECTRA. 


3456755^ 
FIG.  81.  —  Uranium  oxide  (a);  Thorium  oxide  (6);  Cerium  oxide. 

from  that  of  the  Auer  mantle,  being  similar  to  that  of  the  Nernst  glower. 
This  is  no  doubt  due  to  the  greater  thickness  of  the  radiating  layer,  which 
emits  a  more  nearly  saturated  radiation  in  the  region  of  i  to  3  /*,  as  well 
as  in  the  remainder  of  the  spectrum. 


0/234567 
FIG.  82.  —  Beryllium  oxide  (a);  Silica  (fc),  (c);  Feldspar. 


YTTRIUM   OXIDES. 


BERYLLIUM  OXIDE  (BeO);  SILICON  OXIDE  (SiO2);  VANADIUM  OXIDE  (V2O6). 
(Curve  a=BeO;  &  and  c=SiO2;  fig.  82.) 

The  beryllium-oxide  emission  spectrum  is  smooth,  with  two  wide 
maxima  at  3.5  and  5  /*,  respectively.  The  temperature  was  such  that  a 
faint  red  showed  through  the  interstices  of  the  layer  of  white  oxide. 

The  silicon-dioxide  (quartz,  French  flint)  curve  shows  bands  at  2.2, 
2-83,  3-7,  4-4,  and  5.3  /*,  which  coincide  with  the  absorption  bands  (see 
Carnegie  Publication  No.  65,  p.  21).  For  curve  c  the  layer  of  silica  was  1.4 
mm.  For  curve  b  (thinner  layer)  the  surface  temperature  was  about  800°. 


3456 
FIG.  83. — Yttrium  oxide. 

The  vanadium  oxide  in  the  preliminary  examination  showed  no  emis- 
sion bands.    The  substance  is  a  black  powder,  which  melted  at  a  low  red* 
heat,  and  no  further  examination  was  made  into  its  emission  spectrum, 
which  was  similar  to  that  of  carbon.     Curve  d,  fig.  82,  shows  the  emission 
band  of  pure  feldspar  placed  on  the  heater. 


n6 


INFRA-RED   EMISSION   SPECTRA. 


YTTRIUM  OXIDE  (Y2O3). 
(Curves  a  and  6,  fig.  83;  curves  a,  b,  c,  fig.  84.) 

The  surface  color  in  the  two  cases  (fig.  83)  was  a  deep  and  a  bright 
red,  corresponding  to  a  temperature  of  900°  to  1000°.  The  two  curves 
are  similar  in  appearance,  showing  emission  maxima  at  2,  2.76,  3,  3.6,  4.6, 
and  6.9  /*,  respectively,  the  latter  band  being  unusually  sharp.  It  will  be 

70 


2345 

FIG.  84.  —  Yttrium  oxide. 


8/1 


shown  presently,  in  fig.  92,  that  this  sharp  band  may  be  due  to  a  suppres- 
sion of  the  radiation  at  6  /£,  caused  by  a  band  of  metallic  reflection  at  this 
point.  However,  bands  of  metallic  reflection  are  not  common  at  these 
short  wave-lengths,  so  that  the  maximum  at  6.9  tu  may  be  a  true  emission 
band. 

In  fig.  84  are  shown  the  emission  curves  of  three  additional  samples  of 


ERBIUM    OXIDE. 


117 


yttrium  oxide,  of  unknown  purity,  all  of  which  were  a  beautiful  yellow 
color,  as  compared  with  the  sample  given  in  fig.  83,  which  was  a  yellowish- 
white.  These  samples  were  obtained  by  fractional  precipitation  from  the 
sulphate  of  yttrium  by  means  of  oxalic  acid.  The'  precipitation,  however, 
was  not  carried  out  to  the  extent  of  obtaining  yttrium,  erbium,  and  ytter- 
bium oxides  separately.  In  these  curves  the  prominent  bands  of  fig.  83, 
with  maxima  at  2  and  2.75  /£,  are  suppressed,  and  the  small  bands  of  the 
latter  at  3  and  6.8/1  are  the  most  prominent.  These  three  samples  were 
heated  so  that  the  surface  appeared  a  dull  red,  the  sample  for  curve  b 
being  the  hottest.  The  emission  bands  occur  in  two  groups,  at  3  and 
6.8  /*,  respectively,  just  as  was  found  in  the  oxides  of  zirconium  and  mag- 
nesium. There  are  sharp  maxima  at  2.8,  3.15,  and  6.75  /*,  and  smaller 
bands  at  2,  3.9,  and  5.2  ,«,  respectively. 


0/2345678 

FIG.  85.  —  Erbium  oxide. 

ERBIUM  OXIDE  (Er2O3). 
(Curves  a  and  b,  fig.  85.) 

Curve  a  shows  the  emission  of  a  layer  of  the  oxide  formed  by  decom- 
posing a  solution  of  the  nitrate  on  a  strip  of  platinum,  heated  electrically. 
Curve  b  gives  the  distribution  of  energy  of  a  layer  of  the  oxide  on  a  heater- 


n8 


INFRA-RED  EMISSION  SPECTRA. 


tube.  The  two  spectra  are  similar,  with  a  sharp  emission  band  at  2.85  /(. 
Other  maxima  appear  at  2,  3.2,  4.1,  5,  and  7.5  //.  The  general  outline  of 
the  spectrum  is  similar  to  that  of  yttrium. 

NEODYMIUM  OXIDE  (NdO);  MANGANOUS  OXIDE  (MnO). 
(Curve  a,  NdO;  curve  &,  MnO;  fig.  86.) 

The  neodymium  oxide  was  deposited  in  a  thick  layer  upon  a  strip  of 
platinum,  by  evaporation  from  a  solution  of  th»  nitrate.  The  radiation 
curve  shows  maxima  at  3,  4.4,  and  4.83  /*.  Beyond  6  /*  the  emissivity  is 
strong  and  not  unlike  that  of  cerium  and  thorium.  The  scale  of  emissivity 
is  one-half  of  the  curve  for  MnO. 


/4 


10 


'.>  8 


uJ    6 


\ 


01  2  3  4  5  6  7  8JJ, 

FIG.  86.  —  Neodymium  oxide  (a);  Manganous  oxide. 

The  manganous  oxide  was  a  grayish-brown  color.  The  thickness  of 
the  layer  upon  the  "heater-tube"  was  about  1.2  mm.  The  radiating 
surface  was  a  dull  red.  The  spectral  radiation  curve  is  uniformly  smooth 
throughout  its  whole  length,  with  but  a  slight  depression  at  3.2  ft  to  be 
noticed  in  numerous  oxides.  In  these  two  curves  the  sensibility  of  the 
instrument  is  different,  as  is  frequently  the  case. 

ZINC  OXIDE  (ZnO);  LEAD  OXIDE  (PbO). 
(Curve  a,  ZnO;  curve  b  and  c,  litharge  (PbO),  curves  e  and/,  platinum;  fig.  87.) 

The  zinc  oxide  became  a  yellowish-green  on  heating,  resuming  its 
former  white  color  on  cooling.  In  spite  of  this  selective  emission  in  the 
visible  spectrum,  the  distribution  of  energy  in  the  infra-red  is  uniform, 
with  the  usual  depression  at  3.2  //.  Lead  oxide  ("litharge")  melts  at  a  low 
temperature.  On  heating  the  color  changes  from  orange  to  deep  red. 
Curve  6,  fig.  87,  shows  the  distribution  of  energy  for  the  oxide  after  it  had 
melted  into  a  smooth  mass  and  solidified.  Curve  c  shows  the  emissivity 


OXIDES. 


119 


from  an  unmelted  surface.  The  depression  at  3.3  /*  is  marked,  and  at 
5.5  fi  there  is  a  possible  emission  band.  The  thickness  of  the  layer  was  at 
least  1.2  mm. 

Curves  e  and /show  the  emission  of  a  strip  of  platinum.  No  depression 
appears  at  3.3  /*,  from  which  it  would  appear  that  the  depression  at  3.3/1 
is  a  characteristic  of  the  oxides  and  not  due  to  absorption  in  the  instrument. 


18 


16 


12 
'">  10 

'8 

'i  . 


1 


X 

V 


/  2  3  4  5  6  7  QJUL 

Fro.  87.  — Zinc  oxide  (a);  Lead  oxide  (i),  (c);  Platinum. 

IRON  OXIDE  (Fe2O3);  COPPER  OXIDE  (CuO). 
(Curves  a  and  &,  Fe2O3;  curves  c,  d  e,  CuO;  fig.  88.) 

The  iron  oxide  used  was  red  hematite,  or  "  rouge."  The  layer  on  the 
heater-tube  was  1.5  mm.,  heated  to  redness,  800°.  The  energy  curve  is 
smooth,  except  a  depression  at  3.2  /*.  The  copper-oxide  layer  was  1.5  mm., 
heated  to  a  deep  red.  The  energy  curve  is  smooth,  except  the  slight 
depression  at  3.2  ft. 

COBALT  OXIDE  (Co2O3);  CHROMIUM  OXIDE  (Cr2O3);  STANNIC  ACID  (SnO2). 
(Curve  a  =  Co2O3;  curves  b  and  c,  Cr2O3;  curve  d,  SnO2;  fig.  89.     Sensibility  80,  85, 
and  73,  respectively.    Temperature  800°  to  900°). 

The  cobalt-oxide  curve  is  smooth  throughout,  except  the  depression  at 

3P* 

Chromium  oxide  is  green  in  color,  and  emits  a  fairly  smooth  spectrum 
with  a  possible  maximum  at  5  //.     The  depression  at  3.2  /j.  is  prominent. 
Stannic  acid  is  grayish-white  in  color,  but,  unlike  many  of  the  white 


I2O 


INFRA-RED   EMISSION   SPECTRA. 


oxides,  it  emits  a  continuous  spectrum.     The  depression  at  3.2  //  is  small. 
The  transmission  bands  in  cassiterite,  SnO2,  were  found  to  be  small. 

16 


I  2345678/2. 

FIG.  88.  — Iron  oxide,  (a)  (6);  Copper  oxide. 


0/234567  8JLL 

FIG.  89.  —  Cobalt  oxide  (a);  Chromium  oxide  (6),  (c);  Stannic  oxide. 

NICKEL  OXIDE  (NiO);  CALCIUM  SULPHATE  (CaSO4+ 2H2O). 

(Curves  a,  b,  c=  NiO;  curve  d=CaSO4;  fig.  90.     Temperature  about  900°.     Trans- 
mission, Carnegie  Publication  No.  65,  p.  18;  curve  6,  fig.  2.) 

The   nickel-oxide   surface  was  rougher   than   for  the  other   oxides, 
hence   not  so   uniformly  heated.     At   the   highest  temperature  the  de- 


CALCIUM    SULPHATE. 


121 


pression  at  3.3  /*  is  very  marked,  while  at  the  lower  temperatures  it  is 
not  so  deep. 

The  calcium  sulphate  used  was  a  thick,  smooth  layer  of  "plaster  of 
paris,"  which  dehydrated  (at  least  in  part)  at  the  red  heat  used.  The 
emission  bands  at  2,  3.2,  4.65,  and  6.3  /*  coincide  in  intensity  and  position 
with  the  absorption  bands  found  in  previous  work  (see  Carnegie  Publica- 
tion No.  65,  fig.  4).  The  band  at  4.65^  is  shifted  from  its  position  at 


0  I  234567 

FIG.  90.—  Nickel  oxide  (a),  (b),  (c);  Calcium  sulphate. 

4.55  fj.  in  anhydrite  (CaSO4),  but  coincides  with  the  partially  dehydrated 
selenite  (CaSO4+  2H2O)  given  in  Carnegie  Publication  No.  65,  figs.  3  and  4. 
The  absence  of  the  band  usually  found  at  2.8  /£  may  indicate  that  it  is 
not  due  to  water.  Paschen  found  an  emission  band  of  water  at  2.83  /JL  in 
the  Bunsen  flame. 

CALCIUM  OXIDE  (CaO);  TRICALCIUM  PHOSPHATE  [Ca3(PO4)2]. 
(Curves  a  and  &  =  CaO;  curve  c  =  Cas(PO4)2;  fig.  91.) 

The  surfaces  of  these  two  substances  were  at  a  dull  red  heat.  The 
calcium-oxide  layer  was  somewhat  cracked,  but  not  sufficient  to  interfere 
with  the  radiation.  The  layer  of  oxide  in  each  case  was  about  1.2  mm. 

The  calcium-oxide  emission  curve  is  conspicuous  for  its  two  sharp 
maxima,  at  2.8  /JL  and  at  4.75  ju,  respectively,  and  a  high  emissivity  at  8  //, 


122 


INFRA-RED  EMISSION   SPECTRA. 


which  is  not  unlike  that  of  cerium  and  thorium  oxides,  observed  by  Rubens. 
Smaller  bands  appear  at  2.4,  3.3,  and  4  /*.  The  calcium  oxide  was  heated 
to  a  bright  red  before  mounting  it  upon  the  heater-tube,  and  was  apparently 
free  from  the  carbonate  (fig.  92).  Since  there  are  no  emission  bands 
belonging  to  that  substance,  it  appears  that  the  water  used  in  making  the 
CaO  into  a  paste  was  entirely  expelled. 


10 


0/234567 
FIG.  91.  • —  Calcium  oxide  (a);  (6);  Tricalcium  phosphate. 

The  tricalcium  phosphate  has  two  marked  emission  bands,  at  2.85  and 
4.75  /*,  respectively,  and  smaller  bands  at  2  p  and  6.2  fi.  The  emission 
spectrum  is  unusually  similar  to  that  of  apatite  (fig.  75). 

A  chemical  analysis  of  calcium  oxide,  by  Dr.  H.  C.  P.  Weber,  showed 
no  weighable  amount  of  silica.  This  shows  that  the  band  at  2.8  /*  is  not 
due  to  silica. 

CALCITE  (CaCO3). 
(Curves  a  and  &,  fig.  92.     Transmission,  Carnegie  Publication  No.  65,  p.  70.) 

The  sample  examined  was  a  layer  of  finely  ground  white  marble.  The 
surface  color  in  the  two  cases  corresponded  to  about  900°  and  1000°,  re- 
spectively. The  emission  curve  is  of  interest  on  account  of  the  two  types 
of  emission  it  contains.  In  the  region  of  6.7  /z  calcite  has  a  band  of  strong 
selective,  "metallic,"  reflection.  In  this  region  of  the  spectrum  the  emis- 
sivity  is  proportional  to  the  reflecting  power,  thus  placing  in  the  class  with 


CALCIUM  CARBONATE. 


I23 


metals.  Here  the  emission  is  actually  suppressed,  and  we  have  an  emission 
minimum  instead  of  a  maximum,  as  will  be  described  in  the  following 
chapter.  In  the  remaining  part  of  the  spectrum  Ijie  emission  is  propor- 


345 

FIG.  92.  —  Calcite. 


9JJ. 


tional  to  the  absorption.  The  emission  bands  at  2,  2.7,  3.5,  3.9,  4.7,  and 
5.5  JJL  coincide  with  the  absorption  bands  previously  observed.  The  ap- 
parent maximum  at  7.3  jj.  is  due  to  the  suppression  of  the  radiation  at 
6.8  /£.  This  fact  can,  of  course,  only  be  determined  from  a  knowledge  of 


124 


INFRA-RED   EMISSION   SPECTRA. 


the  reflecting  power  of  the  substance  under  examination.  The  maximum 
reflection  and  the  minimum  emission  do  not  coincide  on  account  of  the 
high  reflecting  power  on  the  side  toward  the  long  wave-lengths,  which 
suppresses  the  emission  curve,  thus  shifting  its  minimum  farther  into  the 
infra-red.  The  only  other  example  heretofore  investigated  is  by  Rosen- 
thai,1  for  quartz,  which  will  be  described  in  the  next  chapter.  The  band 
at  4.7  /*  is  of  interest  since  it  occurs  in  CO  anc^  CO2  in  the  vacuum  tube 
radiation  (see  Carnegie  Publication  No.  35),  and  is  in  common  with  the 
carbonates  and  the  sulphates. 

RELATION   BETWEEN   EMISSIVITY  AND   ENERGY   CONSUMPTION. 

The  substances  just  described  must  have  one  or  both  of  two  kinds  of 
spectral  energy  distribution,  due  (i)  to  the  general  absorption  which  is 
present  to  some  extent,  however  small,  and  which  gives  rise  to  a  continuous 
spectrum,  and  (2)  to  bands  of  selective  absorption  which  give  rise  to 
emission  bands. 

The  object  in  examining  the  isochromatic  radiation  curves  of  the 
aforesaid  solids  is  to  determine  whether  or  not  the  observed  sharp  emis- 
sion bands  behave  like  spectral  lines,  or  like  bands  which  include  a  con- 
siderable portion  of  the  spectrum.  If  the  observed  bands  behave  like 
those  of  a  gas,  the  emission  must  be  proportional  to  the  energy  supplied. 

If  the  radiation  is  similar  to 
the  complex  and  highly  damped 
emission  of  a  solid,  e.g.  plati- 
num, the  isochromatics  can  not  be 
straight  lines,  but  must  be  similar 
to  those  of  platinum. 

A  complete  radiator  emits 
energy,  however  small  the 
amount,  of  all  wave-lengths, 
whatever  its  temperature  above 
the  absolute  zero.  The  isochro- 
matic energy  curves  must,  there- 
fore, all  begin  at  the  origin  of 
the  energy  axis;  and  they  may 
have  a  double  curvature.  This 
is  well  illustrated  in  fig.  93  for 
0  '  4  5  watts  tke  isochromatic  radiation  curve 

FIG.  93-  -Isochromatic  radiation  curves  of  platinum.  Q£     platinum     ^      ^^^        The 

platinum  strip  was  50  by  1.5  by  0.02  mm.,  in  an  exhausted  glass  bulb,  with 
a  fluorite  window.  In  this  figure  the  graphs  of  wave-lengths  A=  1.804  p  and 
^=1.968  n  intersect  at  2.8  watts,  showing  that  the  maximum  of  the  energy 
curve  lies  between  these  two  wave-lengths;  temperature  about  1100°  C. 

1  Rosenthal:  Ann.  der  Phys.  (3),  68,  p.  791,  1899. 


EMISSIVITY  AND   ENERGY   CONSUMPTION. 


125 


It  is  interesting  to  note  that  the  point  of  inflection  in  the  curves  oc- 
curs when  the  isochromatic  wave-length  is  identical  with  the  wave-length 
^max>  (Emax)-  This,  of  course,  is  due  to  the  well-knywn  property  of  spec- 
tral emission  of  a  complete  radiator,  or  a  metal,  in  which  the  emissivity 
in  the  short  wave-lengths  increases  more  rapidly  than  on  the  long  wave- 
length side  of  the  maximum  emission. 

On  the  other  hand,  the  graph  showing  the  relation  between  the  emis- 
sivity of  a  spectrum  line  and  the  energy  supplied  should  intersect  the 
energy  axis  at  a  distance  from  its  origin,  corresponding  to  the  energy  (a 
finite  amount)  required  for  excitation,  which  is  different  for  different 
spectral  lines.  This  is  a  well-known  property  of  spectral  lines,  being  inde- 
pendent of  the  wave-length. 

Furthermore,  if  we  follow  the  common  line  of  reasoning  (see  Kayser's 
Spectroscopy,  vol.  II,  pp.  59,  245,  and  331)  and  consider  the  separate 
lines  as  a  part  of  an  energy  curve,  obtained  by  drawing  the  envelope 
through  the  highest  points  of  the  separate  emission  bands,  then  the  maxi- 
mum of  the  envelope  must  shift 
toward  the  short  wave-lengths 
with  increase  in  energy  con- 
sumption, and  the  slant  of  the 
isochromatics  must  be  similar 
to  those  of  platinum  (fig.  93). 
It  is  difficult  to  conceive  how 
this  is  possible  with  discrete 
spectral  lines  which  require  the 
application  of  a  certain  amount 
of  energy  to  excite  them.  The 
change  of  the  emission  curves 
of  the  Nernst  glower  from  a 
discontinuous  into  a  continuous 
one  has  already  been  noticed. 
They  illustrate  this  envelope 
type  of  energy  curve  just  men- 
tioned. But  it  seems  more 
probable  that  this  is  due  to  the 
rapid  growth  of  the  general 
emission  of  the  intervening  fre- 
quencies, which,  with  a  doubtful  broadening  of  the  emission  bands,  oblit- 
erates the  selective  emission  at  high  temperatures.  In  fig.  94  are  shown 
the  isochromatic  energy  curves  of  a  Nernst  glower,  the  values  being 
taken  from  fig.  57.  In  fig.  95  are  given  a  series  of  isochromatics  for  a 
no-volt  glower  1.4  cm.  long  and  1.4  mm.  diameter.  The  current  was 
supplied  from  a  2000- volt  6oo-watt  transformer  on  a  no- volt  circuit. 
The  energy  supply  was  regulated  by  means  of  resistances  in  the  primary. 


80 


70 


60 


50 


40 


30 


20 


10 


024  6  8/0  IZ  WattS 

FIG.  94.  —  Isochromatic  radiation  curves  of  Nernst  glower. 


126 


INFRA-RED  EMISSION  SPECTRA. 


The  voltage  was  obtained  with  a  multiple-cell  electrostatic  voltmeter.  The 
curves  for  the  intensest  part  of  the  spectrum  pass  through  a  double  cur- 
vature and  have  the  general  outline  of  that  of  platinum.  The  normal 
burning  is  80  watts  and  above  that  point  the  isochromatics  appear  to 


1.002,0. 
I.206/O 
I.450,U 
1. 633/0 
2.048/0 
2.752/i 


200 


700 


0  10          20          30         40          50          60  70         80          90      100  Walts 

FIG.  95.  —  Isochromatic  radiation  curves  of  Nernst  glower. 

show  a  slight  increase  in  curvature.  The  intersection  of  the  graphs  for 
wave-lengths  ^=1.206  JJL  and  ^=1.633  /*  at  73  watts1  shows  that  the  maxi- 
mum of  the  spectral  energy  curve  for  this  power  consumption  lies  between 

1  None  of  the  isochromatics,  given   in   this  paper,  have  been  corrected  for  slit-width, 
which  would  change  the  observed  energy  consumption  a  few  per  cent. 


NERNST   GLOWER   ISOCHROMATICS.  127 

these  two  points,  as  previously  observed.  This  method  of  locating  the 
maximum  eliminates  the  correction  for  slit-width,  and  is  an  independent 
proof  of  the  previous  observations  that  the  maximum  of  the  energy  curve 
for  normal  burning  (80  watts)  can  not  lie  at  suck'  short  wave-lengths  as 
was  observed  by  previous  investigators.  This,  of  course,  is  on  the  assump- 
tion that  the  glowers  were  of  the  same  material,  the  base  of  which  is  zir- 
conium oxide  with  a  small  per  cent  of  thorium  or  yttrium  oxide. 

In  Carnegie  Publication  No.  35,  p.  318,  it  was  shown  that  in  the  case 
of  vacuum-tube  radiation  the  intensity  of  the  emission  lines  is  proportional 
to  the  energy  consumption.  The  graphs  there  obtained,  showing  the 
relation  between  current  and  emissivity  of  a  spectral  emission  line,  are 
curved,  due  to  the  fact  that  Ohm's  law  does  not  hold  for  the  vacuum-tube 
discharge.  For  the  present  examination,  instruments  were  available  to 
measure  the  energy  consumption,  when  the  graph  ought  to  be  a  straight 
line,  provided  the  partition  of  the  energy  emitted  is  in  discrete  lines.  As 
a  typical,  selectively  radiating  solid,  oligoclase  was  chosen  on  account  of 
its  sharp  emission  bands,  and  also  on  account  of  its  homogeneity.  A  rod 
2.5  cm.  long  and  2  mm.  diameter  was  prepared  in  an  oxyhydrogen  flame. 
The  spectrometer  slit  was  reduced  to  5  mm.  in  length,  which  permitted 
the  entrance  of  radiation  from  only  about  5  mm.  of  the  central  part  of  the 
rod,  which  was  a  perfectly  clear  glass,  free  from  air-bubbles.  At  the 
highest  temperatures  this  central  part  showed  a  peculiar  faint  white  "lumi- 
nescence" similar  to  the  intense  white  noticeable  in  quartz  when  heated  in 
the  oxyhydrogen  flame.  The  rod  was  thickened  at  the  ends  which  seemed 
to  prevent  internal  reflection  of  the  radiation  from  the  platinum  terminals. 

The  distribution  of  energy  from  the  central  portion  of  this  rod  is  shown 
in  curve  c,  fig.  66,  on  29.4  watts.  The  ends  of  the  0.3  mm.  platinum 
terminals  within  the  glass  rod  were  red  hot.  The  rod  was  viscous,  indi- 
cating a  temperature  of  at  least  1100°  to  1200°;  but  no  light  was  emitted 
other  than  the  hazy  white  glow  already  mentioned,  which  is  in  marked 
contrast  with  the  radiation  from  the  platinum  electrodes.  A  similar 
example  is  given  in  Wood's  Optics,  page  457,  where  it  is  stated  that  sodium 
sulphate,  in  a  loop  of  platinum  wire,  heated  in  a  blast  lamp,  emits  but 
little  light,  although  the  wire  glows  vividly. 

In  fig.  96  are  given  the  isochromatic  emission  curves  of  oligoclase  at 
wave-lengths  2.048,  2.905,  4.445,  an(i  6.082  /*,  respectively,  for  different 
values  of  power  consumption.  The  graphs  are  for  the  same  rod,  under 
the  same  conditions  of  galvanometer  sensibility  and  distance  of  the  radiator 
from  the  silt.  Two  additional  series  of  observations  on  different  days, 
and  using  different  adjustments,  were  made  at  wave-length  2.905  /*.  Only 
one  of  these  graphs  was  parallel  with  the  one  given  in  fig.  96,  showing  that 
there  is  a  variation  in  the  slant  of  the  isochromatic  curve,  under  different 
conditions.  It  will  be  noticed  that,  throughout  the  range  investigated,  the 
change  in  emissivity  is  proportional  to  the  energy  supplied,  just  as  is  true 


128 


INFRA-RED   EMISSION   SPECTRA. 


of  gases.  It  is  possible  that  for  some  wave-lengths  the  thickness  of  the 
radiator  was  not  sufficient  to  emit  a  saturated  radiation,  and  this  may  ex- 
plain why  the  emissivity  at  2.048  /*  apparently  does  not  follow  the  same 
law  as  do  the  other  emission  bands.  In  order  to  have  a  displacement  of 
the  maximum  of  emission,  just  as  is  known  for  solids  emitting  continuous 
spectra,  it  is  necessary  that  the  intensity  of  the  emission  at  the  short  wave- 
lengths increase  more  rapidly  than  it  does  in  the  long  wave-lengths.  The 
2.048  /JL  isochromatic  slants  only  a  little  less  from  the  normal  than  does 


cm 

14- 


16 


18  20  22  24-WaltS 

FIG.  96.  —  Isochromatic  radiation  curves  of  oligoclase. 


X= 6.082  fj..  In  this  region  of  the  spectrum  there  is  a  weak  general  ab- 
sorption, while  the  other  wave-lengths  are  the  maxima  of  selective  emission 
(absorption)  bands,  and  it  is  possible  that  what  corresponds  to  the  emis- 
sivity constant  a  of  a  complete  radiator  is  different  for  the  two  kinds  of 
radiation  found  in  this  substance.  It  is  possible  that  the  isochromatic  at 
2.048  jj.  undergoes  a  sudden  change,  curving  sharply  upward,  at  a  higher 
temperature.  The  same  is  true  of  the  platinum  isochromatic  at  i  /*.  In 
fact,  it  appears  that  the  emissivity  at  2.048  /i  must  suddenly  change  in 
intensity,  unless  oligoclase  is  entirely  different  from  the  other  substances 
examined ;  for,  like  the  others,  in  the  oxyhydrogen  flame,  it  emits  an  intense 


ISOCHROMATICS   OF   OLIGOCLASE.  129 

white  light,  although,  as  shown  in  curve  c,  fig.  66,  the  emissivity  at  2  /* 
is  still  weak  when  the  temperature  is  close  to  the  melting-point. 

In  oligoclase  it  is  not  possible  to  locate  so  definitely,  if  at  all,  the  position 
of  the  maximum  of  the  envelope  by  the  intersection  of  the  isochromatics; 
for  the  lines  show  no  tendency  to  curve,  as  in  platinum,  although  at  closely 
the  same  temperature.  By  the  extrapolation  of  the  2.9  /*  and  the  4.4/4 
isochromatics,  the  intersection  (obtained  after  correcting  for  slit-width)  is 
found  to  be  at  about  8.5  watts.  The  maximum  emission  would  then  lie 
at  about  3.5  /z,  which  is  the  position  of  the  maximum  of  a  complete  radi- 
ator at  850°  abs.  (580°  C.).  On  29.4  watts,  when  the  oligoclase  was 
already  viscous,  indicating  a  temperature  of  1100°  to  1200°  C.  (melting- 
point  1300°),  the  maximum  emission  would  have  to  lie  at  a  much  smaller 
wave-length  than  3.5  /£,  say  at  2  /*,  which  is  inconsistent  with  the  observa- 
tions that  the  emission  curve  is  but  little  different  in  outline  from  those 
obtained  at  lower  temperatures,  —  there  being  no  indication  of  a  shifting 
of  the  energy  distribution.  It  would  therefore  appear  that  it  is  not  per- 
missible to  consider  the  envelope,  drawn  through  the  emission  maxima,  as 
a  criterion  for  judging  the  temperature  of  a  substance  like  oligoclase.  On 
the  whole,  it  appears  that  the  general  emission,  as  distinguished  from  the 
bands  of  selective  emission,  is  less  intense  in  oligoclase  than  in  most  of  the 
other  silicates  studied.  Such  a  substance,  if  it  would  withstand  high  tem- 
peratures, would  most  nearly  approach  the  ideal  light  producer.  For,  since 
the  absorption  coefficient  is  small  throughout  the  region,  to  3  /*,  where,  in 
a  continuous  spectrum,  the  greatest  amount  of  energy  is  emitted,  the  emis- 
sion spectrum  would  remain  discontinuous,  and  only  at  the  highest  tem- 
peratures would  the  general  emission  become  of  importance,  when  a  large 
amount  of  the  energy  radiated  would  be  of  wave-lengths  affecting  the  eye. 

SUMMARY. 

In  general,  the  results  on  the  oxides  furnish  an  excellent  illustration 
of  the  shifting  of  the  maximum  of  intensity  of  emission  toward  the  short 
wave-lengths  with  rise  in  temperature,  just  as  is  known  for  solids  emitting 
continuous  spectra.  In  this  respect  the  various  emission  curves  of  zirco- 
nium oxide  are  particularly  conspicuous.  In  addition  to  what  may  be 
termed  "  general  emission,"  in  which  the  maximum  shifts  with  rise  in 
temperature,  the  curves  of  zirconium  oxide  are  unique  in  having  a  sharp 
band  of  "  selective  emission  "  which  does  not  shift  nor  broaden  with  rise 
in  temperature.  The  results  are  not  unlike  those  obtained  by  Anderson  * 
for  erbium  oxide,  in  the  visible  spectrum.  He  found  that  the  emission 
spectrum  was  not  continuous,  but  consisted  of  bright  bands  superposed 
upon  a  continuous  faint  background.  With  rise  in  temperature  the  bands 
became  more  hazy  in  outline,  and  at  very  high  temperatures  the  spectrum 
became  continuous.  If,  according  to  Stark' s  theory,  the  continuous  spec- 

1  Anderson:  Astrophys.  Jour.,  26,  p.  73,  1907. 


130  INFRA-RED   EMISSION   SPECTRA. 

trum  is  due  to  the  presence  of  a  large  number  of  free  electrons,  this  is  to 
be  expected.  At  low  temperatures  the  electrical  conductivity  is  small,  and 
the  emissivity  is  confined  to  particular  bands  caused  by  certain  groups  of 
electrons.  With  rise  in  temperature  more  electrons,  extending  over  a  wider 
range  of  wave-lengths,  are  excited  to  activity  and  the  separate  emission 
bands  become  merged  into  a  continuous  spectrum.  This  is  not  always 
true,  however,  in  the  present  work.  For  example,  the  sharp  emission  band 
of  zirconium  oxide  at  4.3  /*  seems  to  retain  the  ^ame  intensity,  superposed 
upon  a  continuous  background  of  increasing  intensity,  irrespective  of  the 
temperature.  In  fact,  many  of  the  emission  bands  are  as  sharp  as  those 
found  in  gases. 

Many  of  these  oxides  have  an  emission  band  in  common  in  the  region 
of  2.85  ft,  from  which  it  would  appear  that  this  band  may  be  due  to  the 
oxygen  atom  which  is  in  common  with  all  of  them.  Furthermore,  the 
emission  spectra,  when  smooth,  generally  show  a  depression  at  3.2  //,  for 
which  no  satisfactory  explanation  has  been  found.  There  are  no  atmos- 
pheric absorption  bands  in  this  region,  and  fluorite  is  not  known  (see 
fig.  29)  to  have  absorption  bands  at  this  point  (Paschen's  absorption  curve 
for  a  4  mm.  plate  shows  an  increase  in  absorption  of  2  per  cent  from  2.75 
to  3.2/1,  maximum  at  about  3.1  /*).  The  emission  spectrum  of  platinum, 
carefully  examined  at  the  same  time,  showed  no  depression.  The  emission 
curve  of  the  bare  "heater-tube,"  which  consists  of  a  porcelain  tube  wound 
with  fine  platinum  wire,  likewise  gave  a  smooth  emission  curve  similar  to 
that  of  the  platinum  strip.  On  the  whole,  the  present  data  indicate  that 
the  depression  is  a  characteristic  of  the  oxides.  In  other  words,  the  oxides 
have  a  small  (sometimes  large)  characteristic  emission  band  at  ^=2.8  to 
^=3  /£  and  a  second  group  of  bands  at  4.5  to  5  //.  If  this  be  true,  then  it 
will  be  necessary  to  assign  the  cause  of  the  selective  emission  to  the  element 
common  to  all  the  oxides,  viz,  to  oxygen.  It  is  well  known  that  groups  of 
atoms  have  characteristic  bands;  but,  heretofore,  no  data  has  been  at 
hand  which,  in  any  way,  indicated  the  possibility  of  the  characteristic 
bands  being  due  to  a  particular  atom  in  the  group. 

The  tentative  conclusion  that  these  emission  bands  in  the  oxides  are 
due  to  oxygen  atoms  is  perhaps  to  be  expected,  for  the  presence  of  traces 
of  the  universal  impurity,  silica,  will  not  explain  matters.  The  oxides  are 
the  most  important  and  the  most  stable  of  all  the  important  groups  of 
chemical  compounds;  and  the  spectra  of  substances  belonging  to  a  particu- 
lar group  are  similar.  But  few  substances  are  inert  to  the  action  of  oxy- 
gen. The  spectrum  of  oxygen  (see  Carnegie  Publication  No.  35,  p.  49) 
shows  absorption  bands  at  3.2  and  4.75  /*.  The  emission  spectra  of  CO 
and  CO2  show  bands  in  the  region  of  2.7  and  4.75  fj.  (Carnegie  Publication 
No.  35,  p.  313).  Oxygen  in  a  vacuum-tube  showed  a  strong  emission  band 
at  4.75  /*,  which,  at  the  time  the  research  was  made,  was  ascribed  to  CO 
or  CO2  supposed  to  be  formed  by  the  electrical  discharge.  In  view  of  the 


SUMMARY.  131 

fact  that,  with  rise  in  temperature,  the  CO2  emission  band  shifts  toward 
that  of  CO,  at  4.6  /*,  and  that  eventually  all  three  gases,  CO2,  CO,  and  O, 
when  radiating  by  electrical  excitation  in  a  vacuum-tube,  have  their  im- 
portant emission  band  in  common  at  4.75  /£,  it  does  not  seem  unreasonable 
to  assume  that  the  latter  maximum  is  due  to  the  oxygen  atom  set  free 
during  the  time  intervening  between  dissociation  and  recombination, 
brought  about  by  the  electrical  discharge  in  the  carbon  oxides. 

In  a  broad  sense  the  intensity  and  sharpness  of  the  emission  bands 
are  a  function  of  the  electrical  conductivity.  The  best  insulators  (strong 
bases),  e.g.,  the  refractory  silicates,  the  oxides  of  aluminium,  zirconium, 
erbium,  etc.,  have  the  sharpest  emission  bands,  while  the  best  electrical 
conductors,  such  as  the  oxides  of  cerium,  iron,  zinc,  etc.,  have  no  sharp 
emission  bands.  The  molecular  weight  of  the  base  seems  to  affect  the 
sharpness  of  the  bands  to  as  great  an  extent  as  does  the  electrical  con- 
ductivity, the  sharpest  bands  occurring  as  a  rule  in  oxides  of  low  basic 
molecular  weight.  These  results,  if  true,  are  to  be  expected  from  our 
knowledge  of  the  emission,  absorption,  and  reflection  of  electrical  con- 
ductors and  insulators.1 

Paschen,  using  the  spectral  energy  curves  of  the  oxides  of  iron  and 
copper,  found  no  variation  of  the  emissivity  constant,  a,  with  rise  in  tem- 
perature. On  the  other  hand,  Lummer  and  Pringsheim,  using  the  total 
radiation  from  these  substances,  found  that  the  emissivity  increased  with 
rise  in  temperature.  Such  a  change  in  emissivity  is  to  be  expected,  for  it 
can  be  shown  that  at  the  highest  temperatures  the  emissivity  constant,  a, 
must  decrease,  otherwise  a  point  would  be  reached  where  the  emissivity 
is  greater  than  that  of  a  complete  radiator.  In  the  present  curves,  where 
sometimes  at  high  temperatures  the  emission  seems  to  be  distributed  into 
apparently  two  bands,  the  "constant"  a,  derived  from  the  spectral  energy 
curve  of  one  band,  may  be  different  from  that  obtained  from  the  total 
radiation  measurements.  This  is  illustrated  in  the  present  study  of  the 
Nernst  glower,  where  Mendenhall  and  Ingersoll,2  using  total  radiation, 
found  no  certain  variation  in  a  with  rise  in  temperature. 

From  one  line  of  theoretical  consideration,  one  might  expect  to  find, 
with  rise  in  temperature  and  the  accompanying  increase  in  the  electrical 
conductivity  of  the  oxides,  that  the  reflecting  power  increases.  If  this  be 
true,  then  the  spectral  emission  ought  to  become  more  continuous,  as  is 
found  in  the  Nernst  glower,  and  the  emissivity  constant,  a,  should  retain 
a  high  value  similar  to  that  of  metallic  electrical  conductors.  The  value 
of  a  from  the  spectral  radiation  curve  was  found  to  decrease  with  rise  in 
temperature,  while  Mendenhall  and  Ingersoll  found  no  certain  variation 
in  a,  when  measuring  the  total  radiation.  It  is  evident  that  experiments 

1  This  subject  has  been  thoroughly  treated  by  Aschkinass:  Ann.  der  Phys.  (4),  17,  p. 
960,  1905. 

2  Mendenhall  &  Ingersoll:  Phys.  Rev.,  24,  p.  230,  1907;  25,  p.  i,  1907. 


132  INFRA-RED  EMISSION  SPECTRA. 

on  the  reflecting  power  of  oxides  at  high  temperatures  will  be  necessary. 
The  emissivity  of  the  sharp  spectral  lines  in  a  magnetic  field  will  also 
require  examination,  although,  on  account  of  the  wave-lengths  involved, 
the  possibility  of  obtaining  results  is  not  promising. 

It  may  be  added  that  the  well-defined  maxima  of  emission  are  not 
affected  by  change  in  temperature,  which  is  in  marked  contrast  with  the 
results  of  Konigsberger1  for  the  limited  region  of  the  visible  spectrum. 
Whether  the  emission  maxima  at  2.85  /*  and  4.75  ^  are  due  to  the  presence 
of  water,  in  the  various  minerals,  remains  to  be  determined.  If  they  are 
due  to  water,  then  one  would  expect  to  find  them  in  calcium  sulphate 
(CaSO4+  2H2O),  as  well  as  in  calcium  oxide  (CaO;  probably  some  CaOH). 
But  in  the  sulphate  no  band  was  found  at  2.9  /*,  where  the  emission  should 
be  the  most  intense  if  due  to  water.  The  band  at  4.75  /*  is  characteristic 
of  the  sulphates  (see  Carnegie  Publication  No.  65).  On  the  whole,  the 
assumption  that  these  bands  are  due  to  the  presence  of  water  is  no  more 
satisfactory  than  to  ascribe  them  to  the  common  constituent,  viz,  oxygen. 

The  isochromatics  of  oligoclase  show  that  the  emissivity  is  proportional 
to  the  energy  in-put,  thus  differing  from  the  other  solids  investigated ;  but, 
in  view  of  the  fact  that  in  the  oxyhydrogen  flame  the  oligoclase  emits 
an  intensely  white  light,  it  appears  that  the  emissivity  must  suddenly 
undergo  a  change  in  the  visible  spectrum,  and  perhaps  form  a  more  con- 
tinuous spectrum  in  the  infra-red. 

In  considering  the  emissivity  of  these  oxides  in  connection  with  the 
radiation  from  the  sun  which  is  a  mixture  of  these  substances  in  various 
conditions  of  temperature  and  physical  state,  and  remembering  that  the 
tendency  of  the  solids  is  to  emit  a  continuous  spectrum  at  high  tempera- 
tures, it  forms  an  interesting  field  for  speculation  as  to  what  one  ought  to 
expect  for  the  composite  radiation  from  the  solar  surface.  The  available 
data  show  that  gases  radiating  in  a  vacuum-tube  and  the  metallic  vapors 
in  the  arc  have  their  strongest  emission  lines  in  the  region  of  i  /i.  In  the 
spark  discharge  the  metallic  vapors  appear  to  have  their  maximum  energy 
in  the  ultra-violet.  With  data  provided  by  the  Astrophysical  Observatory 
(see  vol.  2  of  the  Annals) ,  giving  the  distribution  of  energy  in  the  normal 
solar  spectrum,  it  has  not  been  possible  for  me  to  compute  a  consistent  ^max 
for  different  wave-lengths,  nor  a  uniform  value  of  the  radiation  constant  a 
(the  value  of  a  varied  from  21.9  at  0.4  /*,  15.6  at  0.5  /*,  n.o  at  0.6  /*,  7.8  at 
0.7  /j.  to  5.4  at  1.2  //),  by  the  methods  given  on  a  previous  page,  and  it 
seems  evident  that  these  laws  are  not  applicable.  If,  as  the  data  herewith 
presented  on  the  emissivity  of  the  oxides  seems  to  show  (assuming  the  solar 
surface  to  be  solid,  electrolytic  conductor  as  compared  with  a  pure  metal 
with  high  reflecting  power)  these  radiation  laws  do  not  hold,  how  much 
less  must  they  be  true  in  the  case  of  the  solar  surface  in  its  real  condition. 

1  Konigsberger:  Ann.  der  Phys.  (4),  4,  p.  796,  1901. 


CHAPTER  III. 

RADIATION   FROM    SELECTIVELY   REFLECTING   BODIES,  WITH 
SPECIAL   REFERENCE   TO   THE    MOON. 

In  Carnegie  Publication  No.  65,  p.  no,  on  infra-red  reflection  spectra,1 
the  writer  showed  that  all  the  silicates  examined  have  a  region  of  strong 
selective  reflection  in  the  region  of  8  to  10  /*  and  discussed  the  effect  this 
would  have  upon  a  reflecting  surface  like  the  earth,  or  the  moon,  which  is 
composed  largely  of  silicates.  Since  this  discussion  has  brought  forth 
comments  favorable  and  unfavorable,  in  print  and  in  private  communica- 
tions, the  purpose  of  the  present  paper  is  to  summarize,  in  a  general  way, 
the  present  data  bearing  upon  the  subject,  and  to  discuss  certain  points 
not  mentioned  in  the  previous  communication.  The  computed  temper- 
atures of  the  sun  and  of  the  moon  derived  in  the  present  paper  will  be  used 
in  the  subsequent  calculations,  which  will  be  found  to  be  in  slight  disagree- 
ment with  the  writer's  previous  results.  As  a  whole,  it  will  be  shown  how 
easy  it  is  to  obtain  estimated  values  which  ultimately  can  have  but  little 
meaning  other  than  a  guide  in  experimental  work. 

THE   EFFECTIVE  TEMPERATURE   OF  THE  SUN. 

It  has  been  shown  by  Poynting2  that,  when  a  surface  is  a  complete 
radiator  and  absorber,  its  temperature  can  be  determined  at  once  by  the 
fourth  power  law>  if  we  know  the  rate  at  which  it  is  radiating  energy.  If 
it  radiates  what  it  receives  from  the  sun,  then  a  knowledge  of  the  solar 
constant  enables  us  to  find  the  temperature,  which  will  be  the  highest  the 
surface  can  attain  when  it  is  receiving  heat  only  from  the  sun.  Knowing 
the  solar  constant  and  the  radiation  constant  (a=^.^2Xio~5  ergs  Kurl- 
baum)3  Poynting  computes  the  effective  temperature  of  the  sun.  If  s  is 
the  radius  of  the  sun's  surface,  R  is  the  radiation  per  square  centimeter; 
then  the  total  rate  of  emission  is  ^R.  This  must  equal  the  radiation 
passing  through  the  sphere  of  radius  r,  at  the  distance  of  the  earth,  and 
with  a  surface  ^nr2  gives 


where  S  is  the  solar  constant.     Hence  ^  =  46000  S.     Using  the  solar 

1  Investigations  of  Infra-red  Spectra,  Parts  III  and  IV,  published  by  the  Carnegie  Insti- 
tution of  Washington,  D.  C.,  December,  1906. 

2  Phil.  Trans.  Roy.  Soc.  Lond.,  vol.  202  A,  p.  525,  1903. 

3  Kurlbaum:  Wied.  Ann.,  65,  p.  748,  1898. 

133 


134  INFRA-RED  EMISSION  SPECTRA. 

constant  5=2.5  gram  calories  per  minute  =0.1  75  Xio7  ergs  per  square 
centimeter  per  second,  R=  0.805  Xio11  ergs. 

If  we  equate  the  sun's  radiation  to  06*  ,  where  a  is  the  radiation  constant, 
we  get  6,  the  "effective  temperature"  of  the  sun,  that  is,  the  temperature 
of  a  full  radiator  which  is  emitting  energy  at  the  same  rate. 

Thus,  5-35XIO-5  04=  0.805  Xio11;  whence  6=6200°  abs. 

With  each  new  determination,  however,  the  solar  constant  is  being 
reduced  from  the  former  high  value,  so  that  a  more  probable  value1  is 
about  2.1  gram  calories  per  square  centimeter  per  minute. 

Using  this  value  of  S=  2.  1  =  0.147  Xio7  ergs  per  square  centimeter  per 
second, 


whence 

0=5980°  abs. 

This  is  somewhat  closer  to  Wilson's2  value  (5773°  abs.),  which  he 
obtained  by  making  a  direct  comparison  of  the  radiation  from  the  sun 
with  that  from  a  "full  radiator"  3  at  a  known  temperature. 

Warburg4  has  also  computed  the  probable  temperature  of  the  sun. 
He  assumes  that  the  rate  of  radiation  per  degree  is  constant,  and  that  the 
Stefan-Boltzmann  law  is  applicable  at  all  temperatures.  For  5=2.54 
gram  calories  per  second,  the  computed  temperature  is  6256°  abs.,  and  for 
5=2.17  gram  calories  per  second,  a  temperature  of  6014°  abs. 

Fery  and  Millochau5  have  measured  the  temperature  of  the  solar  surface 
with  a  pyrometer.  The  mean  value  of  the  observed  temperature,  after 
correcting  for  atmospheric  absorption,  is  5620°  abs.  For  different  parts 
of  the  solar  disk  the  temperatures  vary  from  5888°  to  5963°  abs. 

Abbot  (loc.  cit.)  gives  a  solar  spectrum  energy  curve,  deduced  from 
bolometric  measurements,  in  which  the  maximum  occurs  at  0.49  //,  with 
a  possibility  of  the  maximum6  lying  at  about  ^=0.46^.  The  equation 
T=  const.  =  2920,  using  the  value  of  A  =0.46^,  gives  a  black-body 


1  Abbot  :  Smithsonian  Miscellaneous  Collections,  vol.  45,  p.  81,  1903.     See  also  his  later 
results,  Annals  Astrophys.  Obs.,  vol.  2,  p.  99. 

2  Wilson:  Proc.  Roy.  Soc.,  vol.  69,  p.  312,  1901. 

3Poynting  (loc.  cit.)  very  aptly  says:  "A  surface  which  absorbs  and  therefore  emits  every 
kind  of  radiation,  is  usually  described  as  'black',  a  description  which  is  obviously  bad  when 
the  surface  is  luminous.  It  is  much  better  described  as  'a  full  absorber'  or  'a  full  radiator,'" 
i.e.,  a  complete  radiator,  as  distinguished  from  a  partial  radiator  or  so-called  "non-black 
body."  In  justice  to  Kirchhoff  who  was  the  first  to  give  a  clear  discussion  of  this  subject,  it 
should  be  called  the  Kirchhoff  radiator. 

4  Warburg:  Verb.  Deutsch.  Phys.  Ges.,  I,  p.  50,  1899. 

6  Fery  &  Millochau:  C.  R.,  143,  pp.  505,  570,  731,  1906. 

6  The  curve  as  drawn  by  Abbot  is  very  asymmetrical  and  depressed  on  the  side  of  the  short 
wave-length.  From  the  data  given,  it  is  possible  to  draw  the  radiation  curve  more  symmetri- 
cal, as  one  would  expect  it  to  be,  which  shifts  the  maximum  to  about  0.46  M.  See  also  his  later 
results,  Annals  Astrophys.  Obs.,  vol.  2,  which  have  been  published  since  writing  this  paper. 


LUNAR  TEMPERATURE.  135 

temperature  of  about  6300°  abs.  One  of  the  most  refractory  substances 
farthest  removed  from  a  full  radiator  in  its  emissive  power  is  bright  plati- 
num. For  this  substance  the  constant  is  2630  instead  of  2920.  If  the 
radiation  from  the  sun  is  purely  thermal  it  must  lie  between  the  "black 
body"  and  bright  platinum  in  its  radiating  properties. 
If  the  sun  radiates  like  platinum,  from  the  equation 

;max  T  =  2630,  using  ;max=  0.46  fi  T=  5700°  abs. 

The  value  of  the  sun's  temperature,  viz,  5900°,  used  in  the  present  compu- 
tations, is  about  the  mean  of  the  "  observed  "  and  the  computed  tempera- 
tures. From  the  results  given  in  the  preceding  chapter,  it  is  evident  that 
the  true  temperature  of  the  sun  can  not  be  determined  by  these  methods. 

THE  LIMITING  TEMPERATURE   OF  THE  SURFACE   OF  THE  MOON. 

Poynting  (loc.  cit.)  further  shows  that  when  there  is  no  conduction 
inwards  from  the  surface,  the  highest  temperature  of  a  full  radiator  is 
attained  when  its  radiation  is  equal  to  the  energy  received.  Equating  the 
energy  to  the  solar  constant,  using  S=  0.175  Xio7  ergs 


whence  0=426°  abs. 

which  is  the  upper  limit  of  the  temperature  of  the  moon,  assuming  that  it 
absorbs  all  the  energy  received  from  the  sun.  If  part  of  the  energy  is 
reflected  and  only  a  fraction  x  of  that  falling  on  it  is  absorbed,  then  the 
effective  lunar  temperature  is  426  -tyx  abs.  From  Langley's1  estimate  that 
the  moon  absorbs  x=l  the  energy  it  receives,  Poynting  (loc.  cit.}  computes 
the  upper  limit  of  temperature  of  the  surface  exposed  to  a  zenith  sun  to  be 

0=426  X  (£)*=4i2°  abs. 

But  this  upper  limit  to  the  temperature  of  the  hottest  part  of  an  airless 
planet  is  never  attained  because  the  moon  turns  the  same  face  to  the 
earth  instead  of  to  the  sun.  He  shows  that,  if  N  is  the  normal  stream  of 
radiation  from  a  unit  of  surface  of  the  moon  immediately  under  the  sun, 
the  normal  stream  from  the  equivalent  flat  disk  is  Nd=%N. 

"  The  effective  temperature  of  the  flat  disk  is  therefore  -\/f  that  of  the  surface  imme- 
diately under  the  sun  at  the  same  distance  from  it.  Then  the  effective  average  temperature 
is  412  X  -\/t  =  371°  abs.  The  upper  limit  then,  to  the  average  effective  temperature  of  the 
moon's  disk,  is  just  below  that  of  boiling  water." 

If  we  use  S  =  0.147  X  io7  instead  of  0.175  X  IO?>  0  =  4°^°  a^s-  an(^  tne 
"effective  average  temperature"  is  408  -\f$~-~l  =  35°°  abs.  or  82°  C.  for 
the  full  moon.  This  assumes  no  conduction  inwards.  Evidently  there 

1  Langley:  Nat.  Acad.  Sci.,  vol.  4,  part.  2,  p.  197. 


136  INFRA-RED   EMISSION   SPECTRA. 

is  some  heat  conducted  inwards,  for  the  results  of  Langley  (loc.  cit.)  show 1 
that  the  surface  of  the  full  moon  is  about  300°  abs. 

FALL  OF  TEMPERATURE  OF  THE  MOON  DURING  ECLIPSE. 
As  already  mentioned  in  Carnegie  Publication  No.  65,  page  no, 
Langley  made  observations  on  the  eclipse  of  the  sun  on  September  23, 
1885,  which  indicate  a  sudden  and  very  rapid  fall  of  energy  received  from 
the  moon  at  the  beginning  of  the  eclipse,  with  some  indications  of  a  rise 
nearly  as  rapid  after  its  conclusion.  In  Carnegie  Publication  No.  65, 
page  113,  are  plotted  his  observed  galvanometer  deflections  during  the 
progress  of  the  eclipse.  The  observations  were  interrupted  by  the  forma- 
tion of  clouds  just  at  the  predicted  time  for  the  moon  to  leave  the  umbra. 
The  curve  shows  that  in  the  short  time  of  about  1.5  hours,  in  passing  from 
the  penumbra  to  the  umbra,  the  radiation  from  the  west  limb  has  fallen 
from  a  maximum  to  a  zero  value.  In  other  words,  the  fall  of  temperature 
is  practically  coincident  with  the  change  in  illumination,  and  at  first  ap- 
peared to  the  writer2  to  indicate  that  the  greater  part  of  the  observed 
energy  is  due  to  reflection.  That  the  moon  at  mid-eclipse  is  still  as  warm 
as  the  earth  is  shown  by  the  fact  that  the  galvanometer  gave  zero  (or  only 
small  positive)  deflections.  If  the  moon  had  been  cooler  than  the  earth, 
then  the  deflections  would  have  been  negative.  Subsequent  search  of  the 
literature  on  the  subject  shows  that  Very3  found  appreciable  radiation  from 
the  moon  during  totality.  The  following  computations  show  that  this  is 
to  be  expected.  After  about  n  days  of  insolation  the  temperature  of 
the  sun-lit  surface  of  the  moon  will  be  fairly  constant.  From  the  surface 
inwards  there  will  be  a  layer  which  may  be  considered  at  a  uniform  tem- 
perature for  the  period  of  1.5  hours,  as  compared  with  n  days.  If, 
then,  the  moon  were  suddenly  eclipsed,  the  fall  of  temperature  of  the 
surface  with  time  (oc=o,  #=/(/);  assuming  at  time  /  =  o,  that  #0  =  300°  abs.) 
is  found  from  the  equation 

,<P0  ,  dd 

k  —  —o6*  =  ds  — 

dx2  dt 

where  k  =  conductivity,  d= density,  and  s=  specific  heat.     The  first  term 
in  the  equation  represents  the  energy  lost  by  conduction  (it  is  assumed 

1  Very,  Astrophys.  Jour.,  8,  pp.  273  and  274,  1898,  however,  records  "inferred  effective 
temperatures"  as  high  as  455°  abs.  for  limited  regions  of  the  moon,  and  (loc.  cit.,  p.  286)  a 
mean  surface  temperature  of  +97°  C.     The  fact  that  the  moon  absorbs  energy  from  the  sun 
(the  maximum  of  which  is  of  short  wave-lengths)  and  emits  energy  of  wave-lengths  from  8  to 
10  /*,  where,  on  account  of  the  probable  high  reflecting  power,  the  emissivity  is  very  low,  would 
explain  why  Mr.  Very  has  found  a  temperature  which  is  much  higher  than  that  of  a  complete 
radiator  under  similar  conditions. 

2  Phys.  Rev.,  23,  p.  247,  1906. 

3  Very:  Prize  Essay  on  the  Distribution  of  the  Moon's  Heat,  p.  40.     Professor  Very  has 
called  my  attention  to  an  error  in  Carnegie  Publication  No.  65,  p.  112,  third  line  from  the 
top:  viz,  "Harrison  (not  Langley)  reminds  the  reader  ..." 


LUNAR  EMISSIVITY.  137 

that  there  is  conduction  in  only  one  direction,  hence  the  differential  terms 
in  y  and  z  disappear).  The  second  term  represents  the  loss  of  heat  by 
radiation  from  the  surface;  while  the  last  term  represents  the  change  in 
temperature  at  the  surface.  The  dependent  variable  enters  here  as  a 
fourth  power,  and  makes  the  solution  difficult.  The  computations  are 
made  for  instantaneous  eclipse,  while  in  the  actual  case  the  shadow  moves 
slowly  across  the  surafce.  Hence  the  error  due  to  neglecting  conductivity 
is  partly  compensated  by  the  slow  eclipse. 

By  neglecting  conductivity  from  the  interior,  the  temperature  of  the 
surface  will  fall  more  rapidly  and  we  have  a  steeper  temperature  decadence 
curve.  The  solution  of  the  equation  is  very  simple  if  we  neglect  conduc- 
tion, and  is 


ds 

for  t  in  minutes,  a=^6.6Xio~12  gram  calories  per  square  centimeter  per 
minute,  d=2,  and  s=o.2  (approximate  values  for  rock  material).     Hence 


300° 


^1+0.0155* 

In  fig.  97  are  plotted  the  temperature  decadence  curves  for  the  radiation 
constant  <r,  for  0.3  a  (emissivity  of  iron  oxide)  and  for  o.i  a.  The  curves 
show  that  for  a  full  radiator  the  temperature  would  fall  from  300°  to  280° 
(room  temperature  when  the  bolometer  would  be  in  temperature  equilib- 
rium with  the  moon)  in  17  minutes,  for  0.3  <r  in  55  minutes,  and  for  o.i  a 
in  140  minutes.  The  solution  for  o.i  a  and  0.3  a  is,  of  course,  only  ap- 
proximate since  the  emissivity  varies  more  nearly  as  the  fifth  power  instead 
of  the  fourth  power,  as  here  used;  and  the  computed  temperatures  should 
be  somewhat  higher.  The  computation  is  also  made  (fig.  97),  assuming 
the  temperature  to  be  350°.  Here,  for  a  complete  radiator  the  temperature 
would  fall  from  350°  to  280°  in  38  minutes,  and  for  0.3  a  in  about  130 
minutes.  By  including  the  conductivity  term  these  periods  would  be  con- 
siderably prolonged.  The  present  solution  is  close  enough,  however,  to 
show  that  the  temperature  (300°)  decadence  curve  is  not  coincident  with 
the  eclipse  curve,  unless  the  emissivity  is  of  the  order  0.3  a.  Hence  the 
writer's1  previous  surmise  that  at  the  beginning  of  totality  radiation  should 
still  be  appreciable  is  substantiated,  although  the  observations  made  by 
Langley  seemed  to  contradict  it.  Very's  observations2  show  that  during 
totality  of  the  eclipse  of  January  17,  1889,  the  radiation  from  the  umbra 
of  the  eclipsed  moon  was  about  i  per  cent  of  the  heat  which  was  to  be 

*  Phys.  Rev.,  23,  p.  247,  1906. 

2  Very:  Distribution  of  the  Moon's  Heat,  p.  40. 


INFRA-RED  EMISSION  SPECTRA. 


20 


300 
Abs 


280 


-0.3  a 


expected  from  the  full  moon.     Boeddicker1  also  records  that  the  minimum 
of  the  heat-effect  falls  decidedly  later  than  the  minimum  illumination. 

REFLECTION  AND  RADIATION  FROM  THE  MOON. 

Using  the  data  just  computed,  we  are  now  in  a  position  to  make  a 

rough  comparison  of  the  relative  amounts  of  energy  reflected  by  and  radi- 

_  .  .ated   from   the    moon.     In 

the  short  wave-lengths  the 
reflecting  power  depends 
upon  the  refractive  index, 
while  at  8  to  10  ft  the  re- 
flecting power  will  depend 
upon  the  refractive  index 
and  the  extinction  coefficient 
(see  Drude's  Optics,  p.  336), 
and  at  all  times  will  have  a 
high  value,  except  at  7  u. 
The  reflecting  power  of  the 
moon  is  variously  recorded 2 
from  0.09  to  0.23,  which  val- 
ues are  several  times  higher 
than  the  refractive  index  of 
ordinary  rocks  permits. 
This  seems  to  indicate  in- 
ternal reflection. 

The  question  is  therefore 
reduced  to  finding  the  ratio 
of  the  energy  emitted  by 
the  moon  to  the  energy  of 
the  sun,  reflected  from  the 
lunar  surface.  For  this  pur- 
pose the  temperatures  of  the 
moon  and  of  the  sun  (350° 
abs.  and  590x5°  abs.,  respect- 
ively) given  on  the  preceding 
pages  are  employed.  The 
values  are  taken  slightly  lower,  although  it  makes  but  slight  difference  in 
the  final  computation.  We  are  not  so  much  concerned  with  the  question 
as  to  where  the  maximum  emission  of  the  moon  occurs  as  we  are  in  the 
fact  that,  in  the  region  of  8  to  10  /*,  where  Langley  observed  radiation  from 
the  moon,  there  is  also  reflected  energy  from  the  sun.  From  the  transmis- 


26C 


240 


220 


200 


20       4O       60       80        100       120    /40minufes 
FIG.  97.  —  Lunar  temperature  decadence  during  eclipse. 


1  Boeddicker:  Trans.  Roy.  Dublin  Soc.,  3,  p.  321,  1885. 

2  Very:  Astrophysical  Jour.,  8,  p.  276,  etc.,  1898. 


LUNAR  EMISSIVITY.  139 

sion  curves  of  the  water  vapor  (curve  c,  fig.  102)  it  will  be  seen  that  in  the 
region  from  5  to  8  /JL  nearly  all  the  energy  will  be  absorbed  by  the  earth's 
atmosphere.  The  reflecting  power  of  the  moon  for  yisible  rays,  according 
to  Langley  (loc.  cit.),  is  only  -g^innnj  frdl  sunlight.  Assuming  that  at  g  fi 
the  reflecting  power  of  the  silicates  is,  on  an  average,  10  times  that  at  0.5 
to  4  //,  this  value  becomes  -5-5%-$-$  or  0.00002.  (This  seems  a  fair  estimate 
of  ratio  of  the  reflecting  power  of  the  silicates  at  i  and  9  /£.  In  the 
absence  of  better  data  this  solution  can  be  only  a  rough  approximation.) 
Using  the  above  values  for  the  temperature  of  the  moon  and  of  the 
sun,  from  Planck's1  formula  for  the  distribution  of  the  energy  in  the  spec- 
trum of  a  complete  radiator  (which,  of  course,  the  moon  is  not) 


we  can  obtain  the  ratio  of  the  intensities  for  the  two  temperatures  7^=350° 
abs.  and  T2=  5900°  abs.  from  the  formula 


Where  £2=14,500  and  A=  9  /*.  The  ratio  is  0.00316.  But  the  moon,  not 
being  a  perfect  radiator,  will  have  a  smaller  emissivity  at  8  to  io/£.  If 
its  surface  were  iron  oxide,2  its  emissivity  would  be  only  0.3  that  of  a  full 
radiator,  and,  for  the  region  at  9  /*,  judging  from  the  drop  in  the  emission 
curve  (see  Carnegie  Publication  No.  65,  p.  in)  and  the  high  reflecting 
power  of  the  silicates,  the  emissive  power  may  be  less  than  this,  say  o.i. 
This  ratio  of  the  emissive  power  of  the  moon  to  that  of  the  sun  will  then 
be  0.000316,  which  is  16  times  (0.000316^0.00002)  the  reflected  energy 
of  the  sun  from  the  moon.  If  we  had  taken  300°  as  the  temperature  of 
the  moon,  then  this  ratio  would  be  (0.00014-^0.00002)  =  7  instead  of  16. 
Computations3  like  these,  which  require  all  sorts  of  assumptions,  can  be  of 
little  value  ultimately.  Any  computation  can  not  be  more  than  a  rough 
approximation,  for  the  reflecting  powers,  observed  up  to  4  /*,  will  be  too 
high,  due  to  internal  reflection.  In  the  region  of  8  to  10  /*  (for  silicates) 
there  can  be  but  little  if  any  internal  reflection.  Hence,  the  ratios  just 
obtained  are  too  low,  but  how  much  so  is  difficult  to  estimate  because 
of  the  lack  of  data.  We  know  that  Langley  observed  also  direct  radia- 
tion from  the  sun,  in  this  region  of  the  spectrum,  and  from  existing  data 
of  the  radiation  from  the  moon  in  this  region  we  do  not  know  how  much  of 
it  is  selectively  reflected  energy  from  the  sun.  The  amount  reflected  must 
be  small,  but,  since  the  total  amount  emitted  is  also  small,  it  is  important 
to  establish  the  fact  that  there  is  selective  reflection  in  this  region. 

1  Planck:  Verb.  Deutsch.  Phys.  Ges.,  2,  p.  202,  1900. 
8  Kayser:  Spectroscopy,  vol.  2,  p.  80. 

•Very:  Astrophys.  Jour.,  24,  p.  353,  1906,  computes  a  much  greater  difference  between 
the  amount  reflected  and  the  amount  emitted. 


140 


INFRA-RED  EMISSION   SPECTRA. 


EMISSION  FROM  A  PARTIAL  RADIATOR. 

We  have  now  to  consider  a  problem  which  has  heretofore  not  been 
discussed  in  connection  with  the  radiation  from  the  moon.  Planck's 
formula  for  the  intensity  of  radiation  at  a  given  wave-length  A  in  the  spec- 
trum of  a  complete  radiator  is 


For  any  radiating  body  the  emission  ^^  =  A^E^  where  A  is  the  absorption 
coefficient.  But  Ax=i—  R^  (for  "transparent  media,"  i.e.,  non-metals), 
where  R  is  the  reflecting  power,  and  hence  0^=^  (i  —  R^. 


23456739        IOJLL 
FIG.  98.  —  Emission  spectrum  of  copper  oxide  and  of  quartz,  325°  C.  (Rosenthal). 

It  follows  therefore  that  a  selectively  reflecting  body  like  quartz  will 
be  a  partial  radiator1  and  that  if  we  compare  its  radiation  curves  with 
that  of  a  complete  radiator,  there  will  be  minima  of  emission  in  the  region 

1  Partial  radiators  may  be  divided  into  so-called  "gray  bodies"  and  bodies  having  a 
highly  selective  emission.  Quartz  belongs  to  the  latter  class. 


PARTIAL   RADIATORS. 


141 


of  8.5  and  9.03  fi.  This  was  predicted  by  Aschkinass1  and  experimentally 
verified  by  Rosenthal,2  with  quartz,  mica,  and  glass  (see  fig.  98).  The 
latter  found  the  observed  emission  curves  of  these  ^ubstances  to  coincide 
precisely  with  the  computed  curves,  using  the  observed  reflecting  power 
of  these  minerals  and  the  observed  emission  curve  of  a  full  radiator  at  the 
same  temperature. 

Since  we  have  constantly  before  us  examples  of  selective  emission,  such 
as  the  Welsbach  mantle,  and  high  temperature  radiation  of  such  metals 
as  tungsten  and  tantalum,  in  the  visible  spectrum,  it  will  be  of  interest  to 
consider  the  radiation  of  several  substances  having  bands  of  selective 
reflection  in  the  infra-red.  In  fig.  99  is  given  an  illustration  of  the  marked 
effect  of  selective  reflection  upon  emission.  The  extraordinary  reflection 
of  carborundum  (SiC)  has  been  found  in  only  one  other  substance,  viz, 


50 

E 


40 


20 


10 


7 


100% 

90 

80 

73 

*>f 

«r 


50 
I 


20 


IO 


10 


13 


14 


FIG.  99.  —  Reflection  from  carborundum  (a);  Radiation  of  complete  radiator  at  300°  abs.  (6); 
Radiation  from  carborundum  at  300°  abs.  (c). 

quartz,  and  in  the  latter  it  is  the  result  of  two  well-defined  bands,  at  8.5 
and  9.03  ft.  The  effect  of  such  a  band  on  the  dispersion  of  this  substance 
must  be  very  marked.  In  fact,  the  unusual  dispersion  in  the  visible  spec- 
trum found  by  others  no  doubt  is  greatly  influenced  by  this  band.  In  this 
figure  are  given  the  computed  energy  curve  for  a  perfect  radiator  (using 
Planck's  formula —  any  temperature  might  have  been  selected  instead  of 
300°  abs.)  and  the  computed  energy  curve  of  carborundum  at  the  same 

1  Aschkinass:  Verb.  d.  Deutsch.  Phys.  Ges.,  17,  p.  101,  1898. 

2  Rosenthal:  Ann.  der  Phys.  (3),  68,  p.  791,  1899. 


142 


INFRA-RED   EMISSION   SPECTRA. 


temperature,  using  the  formula  (fr^E^i—Rj),  where  R  is  the  observed 
reflecting  power  and  E  is  the  corresponding  intensity  of  the  full  radiator. 
It  will  be  noticed  that  the  radiation  will  be  highly  selective,  and  that  at 
10  /J.  it  approaches  closely  to  that  of  the  complete  radiator.  If,  then,  one 
were  to  observe  the  emission  curve  of  carborundum,  the  maximum  at  10  /£ 
would  appear  as  an  "emission  band,"  but  its  explanation  is  different  from 
that  of  emission  bands  of  hot  vapors,  e.g.,  CO2  and  H2O  in  the  Bunsen 
flame.  In  fig.  100  is  given  the  observed  reflection  curve,  <z,  of  quartz,  and 
the  computed  emission  curves  of  a  complete  radiator,  b,  and  of  quartz,  c, 
at  a  temperature  of  400°  abs. 


5  6  78  $  10  II  IZfJL 

FIG.  100.  —  Reflecting  power  of  quartz  (a);  Emission  of  complete  radiator  at  400°  abs.  (&);  Emission 
curve  (c)  of  quartz  at  400°  abs. 

In  fig.  101  are  given  the  computed  emission  curves,  a  and  6,  for  a  com- 
plete radiator,  for  temperatures  300°  and  400°  abs.,  respectively,  and  the 
corresponding  emission  curves,  c  and  d,  of  quartz,  at  the  same  temperature 
using  the  values  of  the  reflecting  power  of  quartz  given  by  Rosenthal 
(loc.  cit.).  These  values  are  somewhat  lower  than  found  by  the  writer. 
In  general,  the  difference  in  the  emissivity  of  the  silicates  and  that  of  a 
full  radiator  at  8  to  10  //  would  not  be  so  marked  as  in  quartz.  For  mica 
(see  Carnegie  Publication  No.  65)  the  emission  minima  would  occur  at 
9.1  and  9.8  //,  while  for  granite  the  emission  minimum  would  extend  from 
8.5  to  10  fjL.  The  combination  would  have  an  appreciable  effect  upon  the 
radiation  curve  of  a  surface  like  that  of  the  moon.  Even,  if  we  exclude 
atmospheric  absorption,  the  emission  curve  (a,  fig.  102)  will  not  be  smooth 
and  continuous,  as  some  writers  seem  to  think.  If  we  superpose  atmos- 
pheric absorption,  the  emission  curve  of  quartz  (curve  c,  fig.  101)  will  be 
somewhat  as  shown  in  curve  b,  fig.  102.  Transmission  curve  c  (fig.  102)  is 


PARTIAL   RADIATORS. 


143 


5  6  7  a  9  10  n 

FIG.  101. —  Emission  of  complete  radiator  (a)  and  (6);  Emission  of  quartz  (computed). 

for  a  column  of  air  no  meters  in  length,  containing  1.2  mm.  precipitable 
water,1  and  is  due  to  Langley  (loc.  cit.).  A  thick  layer  would  shift  the 
maximum  at  8  ft  toward  8.3  /*  found  in  the  moon's  radiation  curve.  In 
fig.  103,  curves  a,  c,  and  d  show  some  of  Langley's  observed  lunar  radia- 

/00% 


5  6  7  3  9  10  II  I2JU. 

FIG.  102.  —  Emission  of  quartz  (curves  a  and  6);  transmission  of  air  (Langley). 

tion  curves,  and  as  a  whole  there  is  a  close  parallelism  between  the  theo- 
retical curve  6,  from  fig.  102,  and  the  observed  curves,  at  10.7  //,  where  we 
have  to  consider  only  atmospheric  absorption.  From  8  to  9  //,  however, 

1  Apparently  water-vapor  is  more  transparent  than  a  film  of  the  liquid,  for  a  film  of  even 
one-tenth  this  thickness  is  opaque  to  heat  rays  beyond  5  /«. 


144 


INFRA-RED   EMISSION   SPECTRA. 


we  have  to  consider  the  combined  effect  of  atmospheric  absorption,  of 
incomplete  emission  of  the  moon  (emission  minima  from  8.5  to  9.8/1), 
and  of  selectively  reflected  energy  from  the  sun.  The  latter  will,  no  doubt, 
vary  the  most  in  intensity.  The  computed  emission  curve  is  the  most 
intense  at  10.2  //,  while  the  observed  is  the  most  intense  at  8.3  /JL  (see  fig.  103). 
This  is  to  be  expected  if  the  observed  energy  curve  is  the  composite  of  the 
selectively  emitted  energy  of  the  moon,  and  the  selectively  reflected  energy 
of  the  sun,  which  is  selectively  transmitted  *by  the  earth's  atmosphere. 
The  selectively  reflected  energy  of  the  sun  would  to  a  certain  extent  fill  up 
the  minima  in  the  lunar  emission  curve,  thus  making  it  higher  at  8.3  /* 
than  at  10.2  /*,  and,  as  far  as  our  present  knowledge  goes,  would  explain 
the  observed  curves  a,  c,  d,  fig.  103,  which  lack  a  minimum  at  8.5  //.  As 


10 
to 

!' 

X' 

C 
2 
Z 

s\ 

20 
10 

a 

b. 

\ 

// 

1  ' 

^ 

N 

/  / 

t 

XN^^ 

±v 

. 

^^ 

''  d/r 

^<£^" 

^^^ 

N 

X 

v^ 

—  '"' 

/// 

v-y 

^v 

56  759/0 

FIG.  103.  —  Emission  of  quartz  (6);  Emission  from  moon  (Langley). 

a  whole,  from  whatever  standpoint  we  view  this  matter  we  come  to  the 
same  conclusion,  viz,  that  in  the  region  from  8  to  10  //  the  energy  emitted 
from  the  moon  consists  of  its  own  proper  radiation  and  of  reflected  energy 
from  the  sun. 

To  sum  up,  we  know  that  Langley  observed  radiation  from  the  moon 
in  the  region  of  8  to  10  //.  He  observed  also  direct  radiation  from  the  sun 
in  this  same  region;  but  we  do  not  know  how  much  of  this  is  superposed 
upon  the  direct  radiation  from  the  moon  due  to  the  latter  being  selectively 
reflecting  in  this  region  of  the  spectrum.  As  stated  in  Carnegie  Publica- 
tion No.  65,  page  115,  computations  which  require  all  sorts  of  assump- 
tions will  not  settle  the  question.  Bolometric  comparison  of  the  spectrum 
energy  curves  of  the  sun  and  of  the  moon  made  at  high  altitudes  will  be 
of  greater  service  in  clearing  up  the  matter.  Very's1  suggestion  of  search- 
ing for  a  solar  image  in  the  lunar  image,  with  a  delicate  heat-measuring 
instrument  covered  by  a  screen  with  a  pin-hole  aperture,  also  deserves  a 
thorough  trial.  Exception  has  been  taken  to  the  writer's  statement  (loc. 
cit.)  that,  of  the  radiation  observed  by  Langley  in  the  lunar  spectrum  at 

1  Very:  Astrophys.  Jour.,  24,  p.  353,  1906. 


LUNAR  RADIATION.  145 

8  to  10  ft,  we  do  not  know  how  much  is  selectively  reflected  energy  from 
the  sun.  But  the  writer  maintains  that  just  as  soon  as  we  admit  the 
possibility  of  the  moon  being  selectively  reflecting  in  the  region  where  it 
has  its  own  proper  radiation,  then  the  use  of  glass1  as  an  absorbing  screen 
for  testing  the  quality  of  that  radiation  is  inadmissible,  although  it  does 
serve  as  a  rough  test  of  the  reflected  energy  at  0.5  to  3  /JL.  For  this  purpose 
it  has  been  serviceable,  and  when  we  consider  the  great  difficulties  under 
which  such  work  must  be  carried  on,  and  the  numerous  corrections  that 
must  be  introduced  (which  at  all  times  may  be  larger  than  the  one  for 
selective  reflection),  the  use  of  a  glass  screen  is  not  objectionable. 

Furthermore,  we  have  noticed  that  the  silicates  reflect  as  a  transparent 
medium  in  all  regions  of  the  spectrum  considered,  except  from  8  to  10  //, 
where  it  reflects  like  a  metal.  This  means  that  the  reflected  energy  in  all 
these  regions  except  8  to  10  /*  will  consist  of  two  parts,  viz,  that  reflected 
from  the  outer  surface  and  that  due  to  internal  reflection.  From  8  to  10  j* 
there  will  be  but  little,  if  any,  energy  due  to  internal  reflection.  Hence, 
to  use  the  reflected  energy  spectrum  of  the  moon  from  0.5  to  4  /*  as  a  means 
of  estimating  the  amount  of  reflected  energy  at  8  to  10  /x  is  not  a  fair  test, 
and,  as  used  (for  want  of  better  data)  in  the  present  paper,  can  only  serve  as 
an  approximation.  As  in  the  case  of  emission  spectra,  it  ought  to  be  possi- 
ble to  analyze  a  substance  by  means  of  its  reflection  bands.  The  constitu- 
tion of  an  isolated  body  like  the  moon,  shining  by  reflected  light,  might  thus 
be  determined.  This  will  probably  never  be  possible  since  terrestrial  atmos- 
pheric absorption  will  interfere  with  the  observations,  while  a  far  greater  sen- 
sitiveness in  the  radiometers  will  have  to  be  attained  than  now  is  possible. 

From  whatever  standpoint  we  view  the  matter,  the  conclusion  is  that 
the  lunar  surface  must  be  diffusively,  selectively  reflecting,  however  small. 
Hence,  in  the  stream  of  energy  reflected  in  any  direction  from  the  lunar 
surface  the  density  will  be  greatest  for  the  wave-lengths  of  the  bands  of 
selective  reflection.  One  would,  therefore,  expect  to  detect  this  difference 
in  all  directions,  and  not  simply  at  the  angle  of  reflection  as  suggested  by 
Very  (loc.  cit.}.  It  is  difficult  to  apply  a  thorough  test  which  will  deter- 
mine whether,  and  how  much  of,  the  energy  from  the  moon  is  due  to  emis- 
sion, and  how  much  is  due  to  reflection  of  energy  from  the  sun.  A  rigid 
comparison  of  the  energy  curves  of  the  sun  and  the  moon  in  this  region 
would  be  of  great  value.  The  polarization  of  the  radiation  from  the 
moon  might  also  be  of  use  in  deciding  this  point.  Pfund2  has  shown  that 

1  Various  observers  have  compared  the  total  radiation  from  the  moon  to  that  part  which 
is  transmitted  by  glass.     Glass  being  opaque  beyond  4  /tt  was  assumed  to  absorb  the  proper 
radiation  from  the  moon,  while  the  part  transmitted  by  glass  was  considered  to  be  reflected 
energy  from  the  sun.     Evidently  if  there  is  selectively  reflected  energy  of  the  sun  beyond  4  /A, 
then  it  will  be  superposed  upon  the  direct  radiation  of  the  moon,  and  the  use  of  a  glass  screen 
for  testing  the  lunar  radiation  is  inadmissible. 

2  Pfund:  Astrophys.  Jour.,  24,  p.  19,  1906. 


146  INFRA-RED  EMISSION   SPECTRA. 

the  bands  of  residual  rays  reflected  from  calcite  are  elliptically  polarized. 
Pfliiger1  studied  the  polarized  radiation  from  tourmaline  heated  to  high 
temperature.  Further  than  this,  nothing  is  known  concerning  the  polari- 
zation of  the  radiation  emitted  by  substances  like  the  silicates;  hence  this 
test  might  not  be  very  decisive. 

To  subject  the  above  conclusions  to  experiment  in  which  the  reflection 
of  a  rough  surface  is  to  be  measured  will  be  accompanied  with  difficulties 
because  of  the  smallness  of  the  surfaces  that  "can  be  used.2  In  the  case 
of  the  moon  an  image  of  the  whole,  or  a  greater  part  of  the  surface,  may 
be  projected  upon  the  spectrometer  slit.  This  means  concentrating  radia- 
tion which  comes  from  a  surface  many  miles  in  diameter,  as  compared 
with  a  surface  which,  when  produced  in  the  laboratory,  amounts  to  only 
a  few  square  centimeters. 

1  Pfliiger:  Ann.  der  Phys.,  7,  p.  800,  1902. 

2  Since  writing  this,  Dr.  A.  Trowbridge,  at  the  Washington  meeting  of  the  American 
Physical  Society,  April  24-25,  1908,  described  a  series  of  experiments  on  the  diffuse  reflection 
of  infra-red  energy,  in  which  he  showed  that  powdered  quartz  has  minima  of  diffuse  reflec- 
tion in  the  regions  where  there  are  absorption  bands  (e.  g.,  2.95  /*),  and  reflection  maxima 
at  8  to  9  /x,  just  as  obtains  for  plane  surfaces.    This  is  an  excellent  illustration  of  the  dif- 
ference of  what  corresponds  to  body  color  and  surface  color  in  the  visible  spectrum  (see 
Wood's  Optics,  p.  352).     It  also  illustrates  the  question  of  diffuse  selective  reflection  discussed 
on  a  previous  page. 


APPENDIX  I. 

THE   EFFECT   OF  THE   SURROUNDING   MEDIUM   UPON   THE 
EMISSIVITY   OF  A   SUBSTANCE. 

In  most  of  the  problems  in  radiation  from  substances  the  surrounding 
medium  is  air,  and  no  account  is  taken  of  the  refractive  index,  which  is 
taken  as  unity.  In  a  remarkable  research  on  "The  Formation  of  River 
Ice  with  Special  Reference  to  Anchor  Ice  and  Frazil,"  by  H.  C.  Barnes, 
it  is  shown  that  the  surrounding  medium  plays  an  important  part  in  the 
rate  of  cooling  of  the  earth's  crust. 

In  Canada,  as  well  as  other  localities  having  high  latitudes,  three  kinds 
of  ice  are  observed,  viz,  sheet  or  surface  ice,  frazil  ice,  and  anchor  ice. 

Surface  ice  is  found  only  in  still  water,  and  is  caused  by  the  loss  of 
heat  to  the  cooler  atmosphere,  by  radiation  and  conduction  from  its  sur- 
face. Thickening  of  the  ice-sheet  takes  place  downwards  by  conduction 
of  heat  through  the  ice  to  the  air. 

Frazil  ice  is  the  French-Canadian  term  for  fine  spicular  ice,  from  the 
French  for  forge-cinders,  which  it  is  supposed  to  resemble.  It  is  found  in 
all  rivers  or  streams  flowing  too  swiftly  for  the  formation  of  surface  ice. 
A  dull,  stormy  day,  with  the  wind  blowing  against  the  current,  is  produc- 
tive of  the  greatest  amount  of  frazil  ice,  which,  like  anchor  ice,  has  a 
tendency  to  sink  upon  the  slightest  provocation,  and  to  follow  submerged 
channels  until  it  reaches  a  quiet  bay.  Here  it  rises  to  the  under  side  of 
the  surface  ice,  to  which  it  freezes,  forming  a  spongy  growth,  attaining 
great  thickness;  in  some  cases  the  author  observed  a  depth  of  80  feet  of 
frazil  ice. 

Anchor  ice,  as  the  name  implies,  is  found  attached  or  anchored  to  the 
bottom  of  a  river  or  stream,  and  often  attains  a  thickness  of  5  to  6  feet. 
It  is  also  called  ground-ice,  bottom-ice,  and  ground-gru.  In  a  shallow, 
smooth-flowing  river  we  are  more  likely  to  have  anchor  ice  formed  in 
excess,  whereas  in  a  deep  and  turbulent  stream  we  are  likely  to  have  more 
frazil.  In  a  river  30  to  40  feet  deep  anchor  ice  is  almost  unknown,  al- 
though large  quantities  of  frazil  are  met  with. 

Barnes  remarks  that  — 

The  various  facts  of  common  observation  in  connection  with  anchor  ice  points  to  radia- 
tion as  the  primal  cause.  Thus  it  is  found  that  a  bridge  or  cover  prevents  the  formation  of 
anchor  ice  underneath.  Such  a  cover  would  act  as  a  check  to  radiation,  and  reflect  the  heat- 
waves back  again  to  the  bottom.  Anchor  ice  rarely  forms  under  a  layer  of  surface  ice  covered 
with  snow.  It  forms  on  dark  rocks  more  readily  than  on  light  ones,  which  is  in  accord  with 

147 


148  EMISSION  SPECTRA. 

what  is  known  in  regard  to  the  more  copious  radiation  of  heat  from  dark  surfaces.  Anchor 
ice  never  forms  under  a  cloudy  sky  either  by  day  or  by  night,  no  matter  how  severe  the  weather, 
but  it  forms  very  rapidly  under  a  clear  sky  at  night.  Anchor  ice  is  readily  melted  off  under  a 
bright  sun.  It  seems  highly  probable,  then,  that  radiation  of  heat  supplies  the  necessary 
cooling  to  the  bottom  of  a  river  to  form  the  first  layers  of  ice,  after  which  the  growth  or  build- 
ing up  of  the  ice  is  aided  by  the  entangling  and  freezing  of  frazil  crystals,  which  are  always 
present  in  the  water. 

The  author  found  that  during  rapid  ice  formation  the  water  becomes 
slightly  undercooled  to  the  order  of  a  few  thousandths  of  a  degree,  and 
that  the  ice  which  is  found  is  in  a  very  adhesive  state.  On  the  cessation 
of  cold  weather  the  temperature  of  the  water  rises  slightly  above  the 
freezing-point  and  the  ice  gradually  melts.  Anchor  ice  rises  from  the 
bottom  in  mild  weather  and  also  in  extreme  cold  weather  under  the  in- 
fluence of  a  bright  sun,  when  it  is  dangerous  to  small  boats.  It  is  also 
known  to  lift  and  transport  large  boulders.  On  the  other  hand,  a  bright 
sun  prevents  the  water  from  becoming  undercooled  and  the  formation  of 
frazil  ice.  The  author's  conclusion  that  anchor  ice  is  formed  by  radiation 
rather  than  by  conduction  is  practically  the  same  as  that  of  Farquharson 
in  1841. 

At  the  request  of  the  editor  of  the  Monthly  Weather  Review,  the  writer1 
has  inquired  into  the  aforesaid  conclusions,  which  at  first  seemed  unten- 
able. After  considering  various  experimental  data,  it  appears  to  the  pres- 
ent writer  that  the  explanation  of  Farquharson  and  of  Barnes  accounts  for 
the  observed  phenomena  better  than  any  of  the  other  theories  propounded. 
Thus  the  loosening  of  the  anchor  ice  under  a  bright  sun  is  simple  enough 
from  the  fact  that  water  is  transparent  to  heat-waves  up  to  i  /*.  The 
thickness  of  the  layer  of  ice  that  must  be  melted  in  order  to  overcome  the 
adhesion  to  the  rock  surface  must  be  of  molecular  dimensions.  In  addi- 
tion to  this,  there  is  the  tension  on  the  rock  surface  due  to  the  buoyancy 
of  the  ice,  which  also  tends  to  melt  the  ice.  The  explanation  of  the  for- 
mation of  anchor  ice  is  more  difficult,  and  the  author's  statement  that 
"it  is  not  to  be  supposed,  because  a  substance  like  water  has  been  found 
to  be  highly  opaque  to  the  radiation  from  hot  bodies,  that  it  will  be  the 
same  for  cold  body  radiation"  is  a  little  startling,  and  not  very  clear,  for 
the  transmissivity  of  any  region  of  the  spectrum  is  independent  of  the 
temperature  of  the  source.  However,  if  total  radiation  is  meant,  then 
such  an  interpretation  is  possible.  In  "Investigations  of  Infra-red  Spec- 
tra" (Carnegie  Publication  No.  35),  several  examples,  e.g.,  methyl  iodide, 
are  given,  illustrating  the  latter  case.  There  is  no  evidence,  however,  for 
saying  that  "it  is  probable  that  water  possesses  an  absorption  band  for 
shorter  heat-waves,  but  may  become  perfectly  transparent  for  the  longer 
heat-waves."  It  is  known  that  water  is  exceedingly  opaque  to  heat  rays 
from  4  to  8  fj.  followed  by  a  more  transparent  region  from  8  to  20  //  (this 

1  The  Monthly  Weather  Review,  p.  225,  May,  1907. 


EFFECT  OF  SURROUNDING  MEDIUM. 


I49 


was  found  by  Rubens  and  Aschkinass1  for  water-vapor,  which  behaves 
like  the  liquid  in  its  properties  for  absorbing  heat-rays),  beyond  which 
there  is  great  opacity.  In  fig.  104  is  given  the  transmission  curve  of  a 
column  of  75  cm.  of  water-vapor  (due  to  Rubens  and  Aschkinass,  loc.  cit.)j 
from  which  it  will  be  noticed  that  there  is  a  quite  transparent  region  from 
8  to  15  /£.  They  found  that  the  heat  waves  at  51  /*  were  entirely  absorbed, 
while  at  80  /JL  theory  (see  Drude,  Optik,  p.  359)  predicts  a  band  of  metallic 
reflection.  Moreover,  water  differs  from  most  other  substances  in  that  its 
great  opacity  is  due  to  numerous  small  absorption  bands.  Consequently 
its  absorption  coefficient  is  smaller  than  that  of  a  substance  like  quartz 
which  has  bands  of  metallic  reflection  at  8.5,  9.02,  and  20.75  /*•  Hence, 
there  is  no  objection  to  saying  that  "the  whole  question  of  the  formation 


100% 


90 


70 


.Q  60 


!*> 


30 
20 
/O 


10         II          12         J3         14-         /5         16 
FIG.  104.  —  Transmission  of  water  vapor. 


Id 


19 


20JLJL 


of  anchor  ice  depends  upon  admitting  that  the  long  heat-waves  can  pene- 
trate freely  through  the  water,"  for  the  maximum  radiation  of  a  body  at 
a  temperature  of  o°  C.  lies  in  the  region  of  the  spectrum  extending  from 
wave-lengths  8  to  20  /*,  and  it  is  here  that  water  has  its  greatest  transparency 
for  long  heat-waves. 

It  is  difficult  to  conceive  of  a  more  complex  form  of  radiation  than  the 
one  here  involved.  According  to  Provost's  theory  of  exchanges,  when  two 
bodies  are  at  different  temperatures  the  hotter  receives  energy  from,  and 
imparts  energy  to,  the  colder  by  radiation,  and  vice  versa.  In  the  case  of 
the  river,  when  the  sky  is  clear,  the  water  is  radiating  into  space  which  is 

1  Rubens  &  Aschkinass:  Ann.  der  Phys.  (3),  64,  p.  584,  1898;  also  Ann.  der  Phys.,  65, 
p.  251,  1898. 


150  EMISSION  SPECTRA. 

probably  at  absolute  zero  of  temperature.  The  river-bed  is  radiating 
energy  into  the  water,  and  probably  through  it  into  space.  Leaving  out 
of  consideration  the  special  nature  of  the  emissivity  of  the  two  bodies 
(water  and  river-bed),  it  has  been  established  (see  Drude's  Optics,  p.  462) 
that  the  radiation  from  a  "non-black  body"  is  approximately  proportional 
to  the  square  of  the  refractive  index  of  the  surrounding  medium,  which  is 
transparent,  so  that,  from  this  standpoint,  the  emissivity  of  the  river-bed 
into  the  water  would  be  greater  than  that  of*the  water  into  the  air.  Of 
course,  if  water  were  perfectly  transparent,  its  emissivity  would  be  nil, 
and  the  problem  would  be  less  complex.  Little  is  known  concerning  the 
special  nature  of  these  two  bodies,  but  from  the  fact  that  the  anchor  ice 
separates  so  easily  from  the  river-bed  under  a  bright  sun,  it  is  evident  that 
the  absorption  coefficient  of  rock  material  is  greater  than  that  of  water, 
and  hence,  that  its  emissivity  must  also  be  greater.  Hence,  more  energy 
will  be  radiated  from  the  river-bottom  than  from  the  water,  into  space, 
the  river-bottom  will  become  the  cooler,  and  finally  a  film  of  ice  is  formed 
on  it.  During  cloudy  weather  the  temperature  of  the  water-vapor  in  the 
air  is  equal  to  or  higher  than  that  of  the  water  and  the  river-bottom.  There 
is  then  an  equality  in  the  radiation,  or  an  excess  is  being  emitted  from  the 
clouds  to  the  earth.  A  certain  amount  will  also  be  returned  from  the 
clouds  by  reflection.  Hence,  the  excess  of  radiation  is  toward  the  earth, 
and  since  the  temperature  of  the  clouds  is  above  the  freezing-point  no 
anchor  ice  is  formed. 

To  sum  up,  from  this  elaboration  of  the  author's  explanation,  just 
quoted,  of  the  formation  of  anchor  ice,  it  will  be  seen  that  it  is  not  only 
possible,  but  also  highly  probable  that  the  cause  is  to  be  attributed  to  the 
greater  emissivity  of  the  substances  forming  the  bed  of  the  river,  and  to 
the  greater  transparency  of  water  to  heat-waves  than  is  generally  supposed 
to  obtain  for  that  substance.  It  is  difficult  to  conceive  that  such  a  condi- 
tion can  exist,  but  the  magnitude  of  the  heat  transfer,  required  to  bring 
about  this  ice  formation,  must  be  exceedingly  small,  and  the  explanation 
given  accounts  for  all  of  the  facts  observed. 

On  a  previous  page  the  conclusion  was  reached  that  the  energy  re- 
ceived from  the  moon  will  be  the  composite  of  the  selectively  reflected 
energy  of  the  sun  and  of  the  selectively  emitted  energy  of  the  moon,  which 
is  selectively  transmitted  by  the  earth's  atmosphere.  In  the  same  manner 
the  earth  would  emit  selectively  in  the  region  of  8  to  10  /JL  and  less  than  a 
complete  radiator  at  the  same  temperature.  It  is,  therefore,  hotter  than  a 
complete  radiator  which  emits  the  same  amount  of  energy.  All  these  ques- 
tions are  of  extreme  importance  to  the  meteorologist  who  is  concerned 
with  the  cooling  of  the  earth.  It  may  be  added,  however,  in  conclusion 
that  the  loss  of  radiation  from  the  earth  under  the  above  conditions  must 
be  exceedingly  small.  In  the  same  manner  the  suppression  and  conser- 
vation of  energy  in  the  region  of  8  to  10  /*  must  be  very  small,  so  that  it  is  a 


EFFECT  OF  SURROUNDING  MEDIUM.  151 

matter  of  speculation  whether  the  earth  would  be  at  a  much  lower  tem- 
perature if  its  surface  were  composed  of  material  not  having  bands  of 
selective  emission. 

Rubens  and  Aschkinass  (loc.  cit.)  found  that  aftef  four  reflections  from 
fluorite  surfaces  the  deflection  due  to  solar  energy  is  reduced  to  zero, 
showing  that  the  opacity  of  the  combined  solar  and  terrestrial  envelope  is 
practically  total  for  wave-lengths  24  to  32  {JL.  In  a  recent  communication 
on  the  absence  of  very  long  waves  from  the  sun's  spectrum,  Nichols1  found 
that  the  atmospheric  transmission  for  wave-lengths  ^=5ijW  can  not  be 
greater  than  3  per  cent.  This,  however,  does  not  indicate  that  the  sun 
is  lacking  in  radiation  of  wave-lengths  ^=51  /*,  as  the  title  of  the  paper 
appears  to  imply,  but  that,  whatever  the  intensity  of  the  radiation  emitted 
at  51  /*,  it  is  almost  entirely  absorbed  by  the  earth's  atmosphere. 

1  E.  F.  Nichols:  Astrophys.  Jour.,  26,  p.  46,  1907. 


APPENDIX  II. 

INSTRUMENTS   AND    METHODS   USED   IN   RADIOMETRY. 

There  are  few  fields  of  experimental  investigation  so  beset  with  diffi- 
culties as  the  quantitative  measurement  of  radiant  energy.  This  is  due 
to  the  fact  that  the  radiation  to  be  measured  is  generally  from  a  surface 
of  which  it  is  practically  impossible  to  determine  the  temperature.  The 
measurement  of  radiant  energy  usually  involves  its  transformation  into 
some  other  form,  and  the  receiver  used  for  this  purpose  is  subject  to  losses 
by  heat  conduction  within,  and  by  reflection,  radiation,  and  convection 
losses  from  its  surface. 

As  a  result  of  inquiry  into  the  development  of  the  various  instruments 
and  methods  used  in  measuring  radiant  energy,  viz,  the  radiometer,  the  ther- 
mopile, the  radiomicrometer,  and  the  bolometer  with  its  auxiliary  galvanom- 
eter, the  writer  has  accumulated  extensive  data,  part  of  which  is  included 
here,  with  the  hope  that  it  may  be  useful  to  others  interested  in  the  subject. 

Various  instruments  have  been  devised,  the  sensibility,  or  the  possible 
chances  for  improvement,  of  which  can  be  rated  without  further  investi- 
gation. That  in  many  cases  the  sensitiveness  has  been  overestimated,  will 
be  noticed  in  the  present  paper.  However,  four  instruments,  viz,  the 
radiomicrometer,  the  thermopile,  the  bolometer,  and  the  radiometer,  have 
been  used  extensively  in  radiation  work,  and  in  each  case  the  inventor  has 
found  qualities  which  seemed  to  him  to  render  his  instrument  superior  to 
other  types.  But,  to  the  writer's  knowledge  in  no  instance  have  all  four 
instruments  been  studied  by  any  one  person;  and,  perhaps,  it  should  not 
be  expected.  Each  instrument  requires  a  special  mode  of  handling,  and 
has  peculiarities  which  can  be  learned  and  controlled  only  after  prolonged 
use.  This  is  particularly  true  of  the  radiometer  and  of  the  bolometer 
with  its  auxiliary  galvanometer. 

Having  already  had  considerable  experience  with  radiometers,  one  of 
which  was  the  most  sensitive  yet  constructed,  the  writer  has,  in  this  exami- 
nation, devoted  most  of  his  attention  to  the  bolometer.  The  investigation 
originated  for  the  most  part  from  the  question  whether  the  radiometer 
was  selective  in  its  action,  in  the  region  of  the  short  wave-lengths.  In 
previous  work  it  was  found1  that  the  radiometer  gave  small  deflections  in 
the  violet  spectrum  of  the  arc  where  Snow,2  using  a  bolometer,  found  large 
deflections. 

1  See  Investigations  of  Infra-red  Spectra.     Part  II.  Infra-red  emission  spectra.     (Carnegie 
Publication  No.  35.) 

2  Snow:  Physical  Review,  I,  p.  32,  1893. 

152 


THE   RADIOMICROMETER.  153 

In  the  course  of  the  discussion  it  will  be  noticed,  as  was  previously 
shown  in  a  general  way,  that  each  instrument  has  some  quality  which 
makes  it  useful  for  a  certain  kind  of  work.  For  measuring  very  narrow 
emission  lines  and  determining  dispersion  curves  the  bolometer  is  no 
doubt  the  best  adapted.  For  measurements  requiring  a  larger  receiving 
surface,  the  linear  thermopile  is  the  more  sensitive  and  the  more  precise. 
It  has  the  further  advantage  that  there  is  no  permanent  current.  On  the 
contrary,  the  bolometer  has  a  current  which  heats  the  bolometer  strip  above 
the  temperature  of  the  surrounding  air.  This  causes  air  currents  which 
make  the  zero  unstable.  This  is  not  true  of  the  thermopile.  Less  is 
known  concerning  the  radiometer,  which  rivals  the  bolometer  and  the 
thermopile  in  sensitiveness.  Furthermore,  the  radiometer  is  not  subject  to 
magnetic  perturbations.  Its  window  limits  its  usefulness  to  the  region  of 
the  spectrum  up  to  20  //.  The  fact  that  it  is  not  portable  is  a  very  minor 
objection. 

I.  THE  MICRORADIOMETER. 

Since  we  are  concerned  with  radiation  meters  of  the  greatest  sensitive- 
ness, the  ingenious  device  of  Weber,1  called  the  microradiometer,  deserves 
passing  notice.  The  instrument  is  not  unlike  a  combination  of  a  differ- 
ential air  thermometer  and  Wheatstone  bridge.  Two  arms  of  the  bridge 
consist  of  a  thin  glass  tube  containing  a  bit  of  mercury  at  the  center  with 
a  solution  of  zinc  sulphate  at  the  ends,  into  which  dip  platinum  electrodes. 
The  ends  of  the  bulbs  are  covered  with  rock-salt  windows.  If  radiant 
energy  is  allowed  to  enter  one  of  these  bulbs,  the  air  expands  and  pushes 
the  liquids  toward  the  opposite  bulb.  This  will  change  the  relative  lengths 
of  the  column  of  mercury  and  of  the  solution  between  the  platinum  termi- 
nals, which  means  a  change  in  resistance  in  the  bridge  arm,  and  a  conse- 
quent deflection  of  the  galvanometer.  The  instrument  was  stated  to  be 
sensitive  to  a  temperature  change  of  0.00001°,  and  while  it  is  not  adapted 
to  spectrum  radiation  measurements,  it  might  be  used  in  total  radiation 
work  where  an  elaborate  installation  is  not  convenient.  By  making  the 
receiving  bulb  of  opaque  non-conductive  material  and  by  covering  the 
inside  with  lampblack,  or  platinum  black,  this  would  be  as  complete  an 
absorber  ("black-body")  as  the  thermopile  or  bolometer.  Its  efficiency 
would,  of  course,  depend  upon  the  gas  inclosed. 

II.  THE  RADIOMICROMETER. 

The  radiomicrometer  is  essentially  a  moving  coil  galvanometer,  of  a 
single  loop  of  wire,  with  a  thermo- junction  at  one  end.  This  instrument 
was  invented  independently  by  d'Arsonval2  and  by  Boys.3  The  former 

1  Weber:  Archiv.  Sci.  Phys.  et  Mat.  (3),  18,  p.  347,  1887. 

2  d'Arsonval:  Soc.  Franj.  de  Phys.,  pp.  30  and  77,  1886. 

3  Boys:  Proc.  Roy.  Soc.,  42,  p.  189,  1887. 


154 


RADIOMETRY. 


used  a  loop,  one  part  of  which  was  silver  and  the  other  was  of  palladium; 
the  latter  used  a  junction  of  bismuth  and  antimony,  which  was  soldered 
to  a  loop  of  copper  wire. 

The  sensibility  of  the  Boys  instrument  was  given  as  TiTTToW To  to 
5?-innnnnr  of  i  degree.  From  subsequent  work  with  other  radiation  meters 
in  which  this  high  degree  of  sensitiveness  has  never  been  attained,  it  would 
appear  that  the  sensibility  of  the  radiomicrometer  was  overestimated.  It 
certainly  has  never  attained  the  sensibility  of  Hie  radiometer,  one  example 
of  which,  used  by  Nichols  (loc.  cit.,  Table  V),  was  twelve  times  as  sensitive 
as  the  radiomicrometer  of  Boys.  The  latter  gave  a  deflection  of  a  little 
less  than  i  cm.  per  square  millimeter  of  exposed  vane  for  a  candle  and 
scale,  each  at  a  distance  of  i  meter.  Paschen1  attempted  to  improve  the 
radiomicrometer,  but  out  of  about  fifty  junctions  only  three  were  useful, 
and  these  were  only  three  times  as  sensitive  as  that  of  Boys,  while  the 
period  was  about  40  seconds.  The  long  period  is  not  always  detrimental, 
however,  for  the  radiomicrometer  is  not  subject  to  magnetic  disturbances 
and  is  a  very  useful  instrument  for  work  not  requiring  the  highest  attainable 
sensitiveness. 

The  writer  has  indicated  further  improvements  in  the  instrument  (see 
Carnegie  Publication  No.  65)  and  places  it  in  a  vacuum,  which  increases 
the  sensibility  by  at  least  70  per  cent.  The  instrument  was  about  six 
times  as  sensitive  as  that  of  Boys  for  a  full  period  of  25  seconds.  Para- 
and  dia-magnetism  limited  the  sensitiveness  to  this  value.  The  work 
with  this  instrument  brought  out  the  fact  that  one  is  inclined  to  use  too 
strong  field-magnets,  and  that  further  progress  can  be  made  by  using 
weak  magnets,  or  narrow  strong  magnet,  situated  as  far  as  possible  above 
the  thermo-junction,  so  as  to  avoid  the  effects  of  para-  or  dia-magnetism. 
The  combination  of  the  radiomicrometer  and  the  radiometer  is  feasible, 
although  the  writer  found  its  usefulness  as  limited  as  that  of  the  radio- 
micrometer.  When  wires  can  be  obtained  more  free  from  magnetic 
material  it  will  possible  to  construct  a  more  sensitive  instrument.  It  is 
doubtful,  however,  whether  it  will  ever  surpass  the  bolometer  used  with  a 
galvanometer  of  the  highest  sensibility.  With  the  radiomicrometer,  Lewis2 
was  able  to  investigate  infra-red  emission  spectra  of  the  alkali  metals, 
which  are  weak  in  energy.  Wilson3  and  Julius4  have  used  the  radio- 
micrometer  for  total  and  spectral  radiation  work,  and  have  found  the 
instrument  highly  satisfactory.  The  slow  period  and  lack  of  portability, 
mentioned  by  some  writers,  is  certainly  not  to  be  weighed  against  its 
indifference  to  magnetic  perturbations  and  constancy  of  the  zero  reading. 
Even  a  slow  period  is  less  objectionable  than  a  quick  period  instrument 

1  Paschen,  Ann.  der  Phys.  (3),  48,  p.  272,  1893. 

2  Lewis:  Astrophys.  J.,  2,  p.  i,  1895. 

3  Wilson:  Proc.  Roy.  Soc.,  55,  1894;  $8,  1895;  60,  p.  337,  1896. 

4  Julius:  Handlingen  5»  de  Nederlandisch  Natuur-  en  Geneeskundig  Congres,  1895. 


THERMOPILES. 


155 


with  which  just  as  much  time  is  lost  by  repeating  observations  which  may 
be  affected  by  the  lack  of  constancy  of  the  zero.  The  instrument  is  self- 
contained,  and  where  the  greatest  sensitiveness  is  not,  required,  it  deserves 
a  wider  application. 

TABLE  IV.  —  SENSITIVENESS  OF  RADIOMICROMETERS  AND  RUBENS  THERMOPILE. 


Observer. 

Full 
period. 

Area  of 
vane. 

Deflections  in  cm  /mm2 
candle  and  scale  at  i  m. 

Radiomicrometer: 
Boys  (Phil.  Trans.,  180  A,  p.  1  59,  1  889) 
Paschen  (Wied.  Ann.,  48,  p.  275,  1893) 

sec. 

10 

40 

ww2 
4 

cm. 
0.9 
3 

Lewis  (Astrophys.  Jour.,  2,  p.  i,  1895) 
Coblentz  (Bulletin  Bureau  of    Stand- 
ards 2   p  479   1906) 

20 
40 
of 

1-4 
3 

1-3  (?) 

3'6. 

6  (in  vacuo) 

Thermopile: 
Rubens  (Wied.  Ann.,  45,  p.  244,  1898) 

14 

i6(?) 

16(250  cm.  total  deflection) 
i  mm  =  1.1°  X  io-«  C. 

In  table  IV  are  given  the  various  radiomicrometers  thus  far  described 
and  their  sensitiveness,  which  is  expressed  in  centimeter  deflections  per 
millimeter  of  exposed  vane,  for  a  candle  and  scale  each  at  a  distance  of 
i  meter. 

III.  THE  THERMOPILE. 

From  a  historical  point  of  view  the  thermopile  has  been  in  use  from  the 
very  beginning  of  radiant  energy  measurements,  and  in  the  hands  of  Tyndall 
and  other  pioneers  in  this  domain  rendered  excellent  service  in  spite  of  its 
great  heat  capacity.  For  spectro-radiometric  work,  however,  only  the  linear 
thermopile  of  Rubens1  is  well  adapted.  This  thermopile  consists  of  20  junc- 
tions of  iron  and  constantan  wires  about  o.i  mm.  to  0.15  mm.  thick  (resist 
3.5  ohms),  and  when  used  with  a  galvanometer,  having  a  figure  of  merit  of 
i=  i.4X  io~10  amp.  (period  =  14  sec.),  a  deflection  of  one  scale  division  indi- 
cated a  temperature  change  of  i.i°X  io~e.  A  candle  at  5  m.  gave  a  deflec- 
tion of  about  10  cm.  or  250  cm.  at  i  m.  The  area  of  the  exposed  face  is 
about  o.8X  20  mm.  The  deflections  were  as  rapid  as  a  bolometer,  and  its 
stationary  temperature  was  reached  in  less  time  than  the  single  swing  of 
the  galvanometer  needle.  In  other  words,  its  heat  capacity  was  so  small 
that  it  gave  an  accurate  register  of  the  energy  falling  upon  it.  In  another 
experiment,  using  a  galvanometer  sensitiveness  of  i=5Xio~10  ampere,  and 
the  scale  at  i  m.,  i  mm.  deflection  =2. 2° X  i o~e  C.  The  sensitiveness  is 
the  same  as  that  of  the  best  bolometers  yet  constructed,  while  its  simplicity 
commends  itself  even  in  spectrum  radiation  work. 

The  general  experience  in  this  country,  however,  has  been  that  the 
commercial  instrument  does  not  fulfill  all  the  excellent  qualities  claimed 
for  the  one  originally  described.  The  wires  are  heavier  than  in  the  original 
specifications,  which  makes  the  instrument  sluggish. 


Rubens:  Zs.  fur  Instrumentenkunde,  18,  p.  65;  1898. 


156  RADIOMETRY. 

The  problem  in  thermopile  construction  is  to  have  it  of  low  resistance 
(equal  to  that  of  the  galvanometer) ,  of  low  heat  capacity  and  heat  conduc- 
tivity, and  of  high  thermoelectric  power.  The  latter  requirement  is  fulfilled 
by  using  iron  and  constantan.  The  heat  capacity  can  be  reduced  by 
using  finer  wire,  say  0.06  to  o.i  mm.  diameter,  and  by  making  the  unex- 
posed  junctions  smaller  than  the  ones  to  be  exposed.  The  latter  are 
soldered  with  quite  large  beads  of  silver,  which  are  then  flattened  to  present 
a  large  surface.  The  unexposed  junctions  do  not  need  this,  and  the  small 
bead  formed  by  the  fusion  of  the  two  wires  (with  a  bit  of  silver  solder  if 
necessary)  can  be  hammered  thin,  in  order  to  have  it  radiate  rapidly.  By 
using  finer  wires  the  resistance  will  be  increased  if  the  dimensions  of  the 
Rubens  pile  be  retained.  In  the  commercial  instrument,  at  least  one- 
third  of  the  wire  is  between  the  unexposed  junctions  and  the  binding-posts. 
The  greater  part  of  this  wire  may  be  eliminated  by  making  the  supporting 
frame  narrower,  while  still  retaining  the  original  distance  between  the 
exposed  and  the  unexposed  junctions.  The  elimination  of  this  superfluous 
wire  will  reduce  the  resistance  by  about  one-third. 

COMPARISON  OF  OLD  AND  NEW  FORM  OF  THERMOPILE. 

In  order  to  test  these  conclusions  in  regard  to  the  use  of  finer  wire,  a 
new  iron-constantan  pile  of  20  junctions,  made  of  wire  0.08  mm.  diameter, 
was  ordered  from  the  makers  of  the  original  instrument.  Although  the 
specifications  were  not  completely  fulfilled  (the  frame  was  nearly  the  same 
size  as  the  original,  which  increased  the  resistance  to  9  ohms),  the  sensi- 
tiveness was  1.4  times  that  of  the  old  type  which  has  resistance  of  4.8  ohms 
(wire  about  0.15  mm.).  By  means  of  suitable  switches  the  two  thermo- 
piles were  connected  to  the  same  galvanometer,  having  a  full  period  of 
12  seconds  (i=2Xio~l°  amp.)  and  exposed  to  the  radiation  from  a  Nernst 
heater.  For  all  deflections,  as  large  as  35  cm.,  the  new  thermopile  showed 
no  drift  greater  than  2  mm.,  which  may  be  attributed  to  the  galvanometer. 
On  the  other  hand,  the  zero  of  the  old  thermopile  would  drift  0.5  cm.  in 
a  10  cm.  deflection  to  2.2  cm.  in  a  27  cm.  deflection  and  would  require 
1 6  to  20  seconds  for  the  deflection  to  return  to  its  original  zero. 

The  two  instruments  were  then  tested  in  a  vacuum.  The  sensitiveness 
of  the  old  instrument  was  increased  only  15  per  cent,  while  no  change  in 
sensitiveness  could  be  detected  in  the  new  one,  although  two  distinct  tests 
were  made  on  different  days,  the  pressure  having  been  reduced  to  o.oi  mm. 

The  thermopiles  are  mounted  on  ivory  frames  and  are  covered  with  a 
sheet  of  copper,  one  side  having  a  slit,  the  other  a  funnel-shaped  opening 
(i  by  15  mm.)  in  it.  The  slit  was  covered  and  the  radiation  passed  through 
the  funnel.  The  whole  was  suspended  from  a  rubber  cork  in  a  wide- 
mouthed  bottle,  which  was  exhausted  with  a  mercury  or  a  Geryk  pump. 
The  source  of  energy  was  an  incandescent  lamp.  With  200  ohms  in 
series  with  the  galvanometer  the  deflections  were  about  10  cm.  The  fact 


THERMOPILES.  157 

that  the  sensitiveness  of  these  thermopiles  did  not  increase  appreciably  in 
a  vacuum  is  rather  remarkable.  Brandes  (Phys.  Zeit.,  6,  p.  503)  found 
that  a  single  junction  of  0.02  mm.  wire  became  18  times  more  sensitive  in 
a  vacuum.  Lebedew  (Ann.  der  Phys.,  9,  p.  209)  found  that  a  0.025  mm- 
iron-constantan  junction  when  black  was  7  times,  and  when  bright  25 
times  more  sensitive  at  a  pressure  of  o.oi  mm.  than  for  atmospheric  pres- 
sure. 

THE  PELTIER  EFFECT. 

The  result  of  the  Peltier  effect  is  to  lower  the  temperature  of  the  ex- 
posed junction.  Consequently,  the  thermopile  does  not  give  an  accurate 
record  of  the  energy  received.  The  actual  error  introduced  has  never 
been  determined.  Since  there  is  a  possibility  of  using  the  thermopile  for 
quantitative  work  in  place  of  the  bolometer,  it  is  desirable  to  learn  the 
degree  of  accuracy  of  this  instrument. 

The  rate  of  generation  of  heat  by  the  Peltier  effect  is  proportional  to 
the  current,  while  the  generation  of  heat  on  account  of  resistance  is  pro- 
portional to  the  square  of  the  current.  Jahn1  has  shown  that  the  heat 
generated  by  the  Peltier  effect,  determined  experimentally,  agrees,  within 
experimental  error,  with  the  value  computed  from  the  observed  thermo- 
electric power.  The  value  for  iron-constantan  has  never  been  determined 
experimentally2  but  from  the  work  of  Jahn  it  is  permissible  to  compute 
the  heat  generated  in  the  thermopile  by  using  the  known  thermoelectric 
power  which  is  about  5oXio~8  volts. 

Using  a  galvanometer  of  5  ohms  resistance  and  having  a  figure  of 
merit  of  i=$Xio~10  ampere  per  millimeter  for  a  scale  at  i  m.,  and  an 
iron-constantan  thermopile  of  20  junctions,  wire  0.08  mm.  and  5  ohms 
resistance,  i  mm.  =  2°X  io~e  C.  The  Peltier  effect  in  calories  is  computed 
from  the  formula 

p  _TU    dE 
''   J   '  dt 

where  ^=274;  ^=3Xio~n  c.g.s.;  /=5  sec.;  7=4.2Xio~7  and  dE/dt= 
5oXio~2  c.g.s.  units; 

P=±5Xio~12  gr.  cal. 

The  total  weight  of  the  junctions  is  about  o.oi  gr.  and  the  specific 
heat  is  about  o.i  gram  calorie.  Hence  the  temperature  change  of  the 
exposed  junctions  is: 

c  y  io~~12 

=5°Xio-9  (for  i  mm.  deflection) 


0.1X0.01 


1  Jahn:  Wied.  Ann.,  34,  p.  755,  1898. 

2  Since  writing  this,  it  has  been  found  that  Lecher  (Ber.  Akad.  Wiss.  Wien,  115,  p.  1505, 
1906;  Sci.  Abstracts,  1083,  1907)  has  recently  determined  this  constant  to  be  12.24  gr-  c^- 
per  amp.  hr.,  while  the  value  previously  computed  was  10.5  gr.  cal.  per  amp.  hr, 


158  RADIOMETRY. 

and  since  the  temperature  of  the  unexposed  junctions  is  changed  an  equal 
amount  in  the  opposite  direction  the  total  A/=i°Xio~8.  But  a  deflec- 
tion of  i  mm.  =  2°X  io~8  C.,  hence  the  error  is  i  part  in  200  under  the  best 
theoretical  conditions.  In  practice  the  temperature  sensitiveness  will  not 
be  so  great;  it  will  be  shown  presently  to  be  of  the  order  5°Xio~8,  whence 
the  Peltier  effect  would  cause  an  error  of  i  part  in  500,  or  i  mm.  in  50  cm., 
which  is  as  close  as  one  can  read  such  large  deflections. 

Since  the  Joule  heat  depends  upon  the  square  of  the  current  it  is  negli- 
gible. Further  consideration  of  the  thermopile  as  an  instrument  for 
quantitative  measurements  will  be  found  below  in  connection  with  the 
bolometer. 

It  will  be  noticed  presently  that  prior  to  his  construction  of  the  iron- 
constantan  thermopile,  Rubens  had  used  several  very  sensitive  bolometers, 
all  of  which  were  displaced  by  the  thermopile.  For  exploring  spectra 
with  very  narrow  lines,  the  linear  bolometer  is  probably  better  adapted 
than  the  pile  which,  however,  may  be  covered  with  a  diaphragm,  having 
a  narrow  slit.  For  extreme  sensitiveness  it  equals  the  bolometer,  and  it 
is  a  noteworthy  fact  that  all  the  investigations  in  the  extreme  infra-red 
and  ultra-violet  parts  of  the  spectrum,  where  the  energy  is  weak,  have 
been  accomplished  by  means  of  the  thermopile.  Unless  one  can  build  up 
an  elaborate  permanent  bolometric  apparatus  in  a  room  not  exposed  to 
direct  sunlight,  the  thermopile  will  give  the  more  reliable  readings,  as  far 
as  the  constancy  of  the  zero  is  concerned.  Whether  or  not  the  thermopile 
will  give  a  true  reading  of  the  energy  falling  upon  it  will  depend  upon 
the  manner  in  which  it  is  employed.  It  requires  no  particular  skill  to 
manipulate,  and  is  easier  to  protect  against  temperature  changes  than 
is  a  bolometer  with  its  storage  battery.  The  " drift"  in  bolometers  to  be 
noticed  presently  was  found  in  the  old  thermopile  when  used  with  a  very 
delicate  galvanometer.  This  was  not  due  to  unequal  increments  of  re- 
sistance as  in  the  bolometer,  but  to  thermoelectric  effects  at  the  binding 
screws,  to  the  connecting  wires  moving  in  the  earth's  magnetic  field,  and 
principally  to  the  large  heat  capacity  of  the  junctions.  Most  of  these 
disturbances,  however,  are  small  and  easily  avoided  in  the  Rubens  type  of 
thermopile.  Since  the  bolometer  strips  and  the  balancing  coils  are  of 
dissimilar  material,  it  is  also  as  subject  to  thermoelectric  disturbances. 

IV.  THE  RADIOMETER. 

The  manner  in  which  an  interesting  scientific  toy  can  be  made  to 
serve  a  useful  purpose  is  well  exemplified  in  the  radiometer  of  Crookes,1 
discovered  about  1875.  By  fastening  bits  of  pith  (the  one  black,  the 
other  white)  at  the  ends  of  a  long  straw,  which  was  suspended  by  means 
of  a  silk  fiber  in  a  long  glass  tube,  he  was  able  to  make  measurements  of 

1  Crookes:  Phil.  Trans.  (II),  166,  p.  325    1876. 


THE    RADIOMETER.  159 

radiant  energy,  even  at  that  early  date.  Pringsheim1  simplified  the  instru- 
ment somewhat,  suspended  the  vanes  bifilarly  with  silk  thread,  and  used  it 
to  investigate  the  infra-red  spectrum  of  the  sun  to  about  1.5  /*,  produced  by 
means  of  a  glass  prism.  From  this,  the  first  really  useful  radiometer  was 
developed  by  Nichols.2  The  behavior  of  the  radiometer  has  been  worked 
out  theoretically  by  Maxwell3  in  his  paper  on  "Stresses  in  Rarefied  Gases 
arising  from  Inequalities  of  Temperature."  Among  other  things  he  showed 
that  for  two  parallel  disks  very  near  each  other  the  central  points  will  pro- 
duce but  little  effect,  because  between  the  disks  the  temperature  varies  uni- 
formly, and  only  near  the  edges  will  there  be  any  stress  arising  from  an 
inequality  of  temperature  in  the  gas.  It  has  been  shown  by  others,  especially 
by  Crookes  and  by  Nichols,  that  the  sensitiveness  of  the  radiometer  is  a 
function  of  the  pressure  of  the  residual  gas,  of  the  kind  of  gas  surrounding 
the  vanes,  and  of  the  distance  of  the  exposed  vanes  from  the  window.  The 
latter,  on  account  of  its  absorption,  of  course  limits  the  region  of  the  spec- 
trum which  is  to  be  investigated.  If  the  vanes  are  not  too  close  to  the  win- 
dow, the  deflections  will  be  proportional  to  the  energy  falling  upon  one  of 

them. 

COMPARISON  OF  SENSITIVENESS  AND  AREA  OF  VANE. 

For  veins  of  finite  dimensions,  such  as  must  be  used  in  practical  work, 
the  writer  has  found  that  the  deflections  are  proportional  to  the  area  of 
the  exposed  surface  of  the  vane.  This  is  perhaps  to  be  expected,  although 
there  seemed  to  be  some  doubt.  The  curve  of  deflections  and  exposed 
area  of  vanes  (area  10.5X1.3  mm.  for  constant  pressure  of  0.02  mm.)  does 
not  pass  through  the  origin.  One  explanation  may  be  that  for  infinitely 
narrow  vanes  the  graph  is  not  a  straight  line,  but  curves  as  it  approaches 
the  origin.  Because  of  the  impracticability  of  suspending  vanes  of  different 
widths  successively  from  the  same  fiber  suspension,  at  the  same  distance 
from  the  window,  using  the  same  gas  pressure  for  all  vanes,  it  was  necessary 
to  use  one  wide  vane,  with  a  slit  before  it,  and  vary  the  opening  of  the  slit. 
The  source  of  energy  (Nernst  heater)  was  at  a  distance  of  3  meters  and, 
hence,  the  width  of  the  projection  of  the  slit  upon  the  vane  was  practically 
the  width  of  the  slit,  except  for  a  very  narrow  slit  when  diffraction  may 
decrease  the  energy  incident  upon  the  vane.  This,  however,  would  dis- 
place the  graph  still  farther  from  the  origin.  The  forces  acting  in  a  radi- 
ometer are  so  complex  and  so  little  understood  that  no  further  examination 
was  made  to  ascertain  the  limits  within  which  the  above  proportionality 
holds.  The  test  was  made  to  reduce  the  deflections  to  unit  area  between 
the  above  limits  of  exposed  vane.  It  is  of  interest  to  note  in  this  connec- 
tion that  in  a  bolometer  the  sensitiveness  varies  as  the  square  root  of  the 
area  of  the  bolometer  strip. 

1  Pringsheim,  Ann.  der  Phys.  (3),  18,  p.  32,  1883. 

2  Nichols:  Phys.  Rev.,  4,  p.  297,  1897;  Berl.  Ber.,  p.  1183,  1896. 

3  Maxwell:  Collected  Papers,  2,  p.  681;  Phil.  Trans.,  Part  I,  1879, 


l6o  RADIOMETRY. 

In  his  earlier  communications  the  writer  held  to  the  belief  that  the 
weight  of  the  vanes  and  their  size  were  the  most  important  factors  in 
determining  the  sensitiveness  and  period  of  a  radiometer.  However,  so 
many  factors  enter  into  the  problem  that  it  is  difficult  to  decide  this  point, 
and  the  following  test  may  be  of  interest,  showing  that  the  sensitiveness  is 
determined  principally  by  the  diameter  of  the  quartz  fiber  suspension. 

COMPARISON  OF  SENSITIVENESS  AND  DIAMETE»  or  FIBER  SUSPENSION. 

Using  the  same  vanes  (0.5X9  mm.  area)  the  sensitiveness  and  period 
were  found  for  a  heavy  and  a  light  quartz  fiber  suspension.  The  main 
difficulty  was  to  insure  that  the  vanes  were  at  the  same  distance  from  the 
window,  and  that  the  pressure  was  the  same  in  the  two  cases.  Hence  these 
qualities  are  only  approximate.  In  table  V,  it  will  be  noticed  that  in 
changing  from  a  heavy  to  a  light  fiber  the  sensitiveness  is  increased  4.5 
times  while  the  period  was  increased  almost  threefold.  It  further  shows 
that  for  the  same  (light)  fiber  the  sensitiveness  was  doubled  (36  to  71  cm. 
per  mm.)  by  changing  the  pressure  and  the  distance  from  the  window, 
which  was  of  fluorite,  and  hence  opaque  beyond  10  /*.  The  sensitiveness 
of  71  cm.  per  square  millimeter  of  exposed  area  is  the  highest  on  record. 
This,  however,  was  not  the  maximum  sensitiveness  since  the  pressure  was 
0.02  mm.,  while  radiometers  have  their  maximum  sensitiveness  at  a  pres- 
sure of  about  0.05  to  o.i  mm.  At  this  pressure,  however,  heat  conduction 
would  cause  annoyance. 

The  vanes  of  this  suspension  were  of  platinum  foil  o.oi  mm.  thick, 
covered  on  one  side  electrolytically  with  platinum  black,  and  then  smoked 
over  a  candle.  (It  is  best  to  cool  the  gases  from  the  flame  by  placing  a 
wire  gauze  or  sheet  of  metal  full  of  holes  between  the  flame  and  the  vanes 
when  smoking  them.)  These  vanes  were  suspended  by  means  of  one  of 
the  finest  workable  quartz  fibers,  and  when  within  3  mm.  of  the  window 
either  one  of  the  vanes  would  always  approach  and  adhere  to  it,  even  at 
atmospheric  pressure.  From  tests  with  fluorite  windows,  which  from 
internal  strains  might  be  piezoelectric,  and  with  rock-salt  windows,  when 
bare,  and  also  when  covered  with  tinfoil,  it  was  found  that  this  effect  is 
not  due  to  electrification.  Starting  with  the  vanes  parallel,  and  at  a  dis- 
tance of  about  5  mm.  from  the  window,  it  was  found  that  as  this  distance 
was  decreased  one  of  the  vanes  (generally  the  one  to  be  exposed  to  radia- 
tion) would  approach  the  window,  and  for  a  distance  of  about  3  mm. 
would  turn  until  the  plane  of  the  vanes  was  at  right  angles  to  the  window. 
The  observations  extended  over  several  months,  and  all  evidence  indicates 
that  this  effect  is  due  to  gravitational  attraction.  As  a  result  of  this  the 
deflection  of  such  a  vane  would  not  be  proportional  to  the  energy  received. 
The  radiometer  had  no  torsion-head  to  control  the  zero.  However,  for 
general  work  with  very  sensitive  radiometers  a  torsion-head  would  be  nec- 
essary, since  the  best  pumps  may  leak,  which  will  cause  a  slow  "drift." 


THE   RADIOMETER. 


161 


It  will  be  shown  presently  that  this  is  about  five  times  the  sensitiveness  of 
Snow's  bolometer,  for  which  i  mm.  deflection  (scale  at  3  m.)  indicated  a 
temperature  difference  of  7.5°Xio~6.  In  other  words,  this  radiometer 
would  detect  l  OOQ  000  degree  rise  in  temperature.  But  the  period  of  the 
radiometer  was  6  times  that  of  the  bolometer,  which  is  its  weakest  point  in 
radiation  work  requiring  a  short  period. 

TABLE  V.  —  SENSITIVENESS  OF  RADIOMETERS. 


I 

Pressure. 

Full 
period. 

Deflection  per 
square  mm. 

Remarks. 

mm. 

sees. 

cm. 

O.O2 

45 

7-9 

Vanes  (area  0.5X9  mm.,  weight  5  mg.)  3  mm. 

from  window. 

.02 

60 

w-S 

Vanes  closer  to  window. 

SAME  VANES  (0.5X9  mm.),  FINER  QUARTZ  FIBER. 

mm. 

min. 

cm. 

O.O2 

2 

36 

Vanes  3  mm.  from  window  candle  at  3  m. 

•°3 

2-5 

71 

Vanes  nearer  window.     This  is  the  greatest 

recorded  sensitiveness.     A  deflection  of  i 

mm.  on  scale  at  i  m.  =  3.8°Xio—  °. 

O4. 

63  < 

HEAVY  VANES,  AREA  1.3X10.5  mm.,  WEIGHT  lo+mg. 

mm. 

sees. 

cm. 

O.O2 

40 

5-5 

Distance  of  vanes  from  window  unknown. 

.02 

60  to  64 

13-7 

Vanes  nearer  window,  hence  longer  period. 

.026 

36  to  40 

8-5 

Vanes  farther  from  window  than  in  preceding. 

•°3S 

25 

3-3 

Vanes    still    farther    from    window,    which 

shortens  period  and  decreases  sensitiveness. 

For  a  pressure   of   about   0.05   mm.   the 

sensitiveness  would  be  much  greater. 

A  comparison  can  also  be  made  (table  V)  between  light  vanes  (0.5X9 
mm.)  and  heavy  ones  (1.3X10.5  mm.)  at  the  same  pressure  but  having 
different  quartz-fiber  suspensions.  The  results  show  that  while  the  light 
weight  vanes  are  more  sensitive  than  the  heavy  ones  (see  table  V),  there 
seems  to  be  no  limit  to  the  sensitiveness  attainable  in  either  case,  pro- 
vided one  does  not  consider  the  period.  The  idea  of  not  considering  the 
period  of  vibration  with  sensitiveness  seems  reasonable,  for  by  sensitive- 
ness is  meant  the  minutest  quantity  of  radiation  one  can  detect,  assuming 
one  is  willing  to  wait  for  the  deflection  to  reach  a  maximum.  In  table  VI 
are  compiled  the  most  notable  radiometers  used  in  radiation  work.  The 
use  of  a  candle  as  a  standard  of  comparison  is  questionable,  but  since  the 
sensitiveness  of  the  various  instruments  varies  by  a  factor  from  2  to  20, 
it  is  sufficiently  accurate  for  the  present  comparison.  In  this  table  it  will 
be  noticed  that  for  the  same  period  the  various  radiometers  vary  in  sensi- 


162 


RADIOMETRY. 


tiveness  by  as  much  as  50  per  cent.  Porter's  radiometer  was  the  most 
sensitive  of  the  instruments  having  a  period  of  90  seconds.  But  he  gained 
little,  on  the  whole,  for  the  vanes  were  so  light  that  he  could  work  only 
during  the  quiet  hours  at  night.  On  the  other  hand,  the  writer,1  after 
trying  light  vanes,  adopted  the  heavy  ones,  and  was  thus  able  to  work  at 
all  hours,  even  with  a  large  air-compressor  in  operation,  in  an  adjoining 
room. 

TABLE  VI.  —  SENSITIVENESS  OF  VARIOUS   RADIOMETERS. 


Observer. 

Full  period. 

Area  of  vanes. 

Deflections    per 
square  mm  .  area 
of  exposed  vane; 
candle  and  scale 
each  at  i  m.1 

E,  F.  Nichols: 
Phys.  Rev.,  4,  p   297,  1897  

min.   sec. 
0       12 

;,  sq.  mm. 
2X15 

cm. 

sc?) 

Astrophys.  Jour.,  13,  p.  101,  1901  

O       II 

•2.1 

12.  S 

Stewart: 

I       2O 

•7Q 

A  Q 

Phvs.  Rev  ,  13  ,  D  2^7   IQOI 

5" 

17 

Drew:  (Phys.  Rev     I7»p  321    1903) 

I       3O 

7 

17.  1 

Porter  (Astrophys.  Jour.,  22,  p.  229,  1905)  
Coblentz  

I   30 

I       3O 

3-6 

is  doX  i.O 

27-5 

8  to  10 

Abs.  Spectra 

<?o 

12 

I  O  to  12 

Phys.  Rev.,  16,  20,  and  22.    (Vac.  tube).. 
Another  vane 

I     40 

2       3O 

ii  (nXi) 

A.  f 

52 

71 

(Ibid.)   . 

I       IO 

A.  Z 

7C 

SENSITIVENESS  COMPARED  WITH  WAVE-LENGTH  OF  EXCITING  SOURCE. 

In  his  investigations  of  emission  spectra  of  the  alkali  metals,  using  a 
bolometer,  and  a  prism  and  lenses  of  quartz,  Snow2  found  that  the  vapor 
of  the  carbon  arc  had  the  larger  portion  of  its  energy  concentrated  in  one 
large  band  in  the  violet.  The  writer,3  using  a  radiometer,  a  rock-salt 
prism,  and  a  mirror  spectrometer  for  investigating  infra-red  emission 
spectra,  found  that  the  radiometer  gave  but  small  if  any  deflections  in  the 
violet.  The  violet  band  is  far  enough  from  the  reflection  minimum  of 
silver  not  to  be  weakened  by  it,  hence  it  appeared  that  the  radiometer 
might  be  selective  in  its  behavior  to  radiant  energy. 

To  test  this  point  the  following  experiment  was  tried.  The  total 
radiation  from  an  aluminum  spark  (with  glass  plate  condenser)  on  a 
10,000  volt  transformer  was  measured  with  a  very  sensitive  radiometer 
(period  65  to  70  seconds;  sensitiveness  35  cm.  per  square  millimeter)  just 
described,  and  with  a  bolometer,  to  be  described  subsequently.  The  win- 
dow of  the  radiometer  was  of  white  fluorite  2  mm.  thick,  hence  transparent 
to  the  ultra-violet,  but  opaque  beyond  10 ,«.  A  large  point  of  the  energy 
of  the  aluminum  spark  lies  in  the  ultra-violet.  The  maximum  energy  of 
the  warm  electrodes  occurs  at  about  8  //.  The  radiation  from  the  spark 

1  "Investigations  of  Infra-red  Spectra,"  Carnegie  Publication  No.  35,  1905. 

2  Snow:  Phys.  Rev.,  I,  pp.  28  and  221,  1893. 

3  Carnegie  Publication  No.  35,  Part  II,  1905. 


THE   RADIOMETER.  163 

passed  through  a  quartz  cell  8  mm.  thick,  containing  distilled  water  which 
absorbed  all  of  the  infra-red  energy.  The  observations  consisted  in  ob- 
taining the  ratio  of  energy  transmitted  by  a  glass  plate  8  mm.  thick  (which 
is  opaque  to  rays  shorter  than  0.3  fj)  to  the  total  energy  of  the  spark. 

Unfortunately,  at  the  high  sensitiveness  required  for  ultra-violet  radia- 
tion the  two  instruments  were  not  in  perfect  working  order  at  the  same 
time.  During  the  first  test  the  bolometer  had  a  short  period,  10  seconds, 
and  caused  trouble  by  the  "drifting"  of  the  zero  with  changes  in  the 
spark,  while  in  the  second  test  the  radiometer  was  leaking  slightly,  which 
caused  its  zero  to  "  drift."  Then,  too,  the  spark  was  by  no  means  constant, 
but,  as  will  be  seen  presently,  the  ratio  above  referred  to  is  about  the  same 
for  the  radiometer  and  the  bolometer,  after  correcting  for  the  loss  of  4 
per  cent  by  reflection  at  the  fluorite  window.  The  agreement  is  close 
enough  to  show  that  the  radiometer  is  not  selective  in  its  action  and  hence 
is  adapted  to  investigations  in  the  ultra-violet. 

In  the  first  test  the  direct  radiation  from  the  aluminum  spark  (with 
condenser)  was  compared  with  the  part  transmitted  by  glass.  There  was 
some  infra-red  energy  in  this  case  which  made  the  ratio  lower  than  in  the 
second  experiment.  The  direct  deflections  with  the  radiometer  were 
about  20  cm.  The  ratio  of  the  deflection  through  a  piece  of  plate  glass 
to  the  direct  deflection  varied  from  17  to  19.5  per  cent  (mean  about  18 
per  cent),  while  with  the  bolometer  the  same  ratio  varied  from  1 6  to  20  per 
cent,  the  mean  being  about  19  per  cent.  The  bolometer  followed  the 
fluctuations  of  the  spark,  hence  the  greater  variations.  The  spark  was 
75  cm.  from  the  radiometer  and  25  cm.  from  the  bolometer. 

In  the  second  test  the  infra-red  radiation  was  absorbed  by  a  cell  of 
water,  with  quartz  windows.  Plate  glass  was  again  used  to  absorb  the 
ultra-violet.  In  this  test,  the  ratio  of  the  radiation  transmitted  by  the 
glass  to  the  total  radiation  was  on  an  average  about  65  per  cent,  while 
the  same  ratio  for  the  bolometer  was  67  per  cent.  Correcting  for  the 
loss  by  reflection  at  the  window  the  ratio  for  the  radiometer  would  be 
about  69  to  70  per  cent. 

The  results  show  that  the  radiometer  is  not  selective,  i.  e.}  it  is  as  efficient 
in  the  ultra-violet  as  in  the  bolometer. 

THE  RADIOMETER  COMPARED  WITH  THE  BOLOMETER. 

In  discussing  the  merits  of  the  radiometer,  writers  have  generally 
emphasized  the  fact  that  the  radiometer  is  not  adapted  for  quantitative 
work  since  it  cannot  be  calibrated.  As  a  matter  of  fact,  in  reviewing  the 
work  done  in  radiation  it  was  found  that  even  with  the  bolometer  there 
are  only  a  few  cases  where  the  energy  was  obtained  in  absolute  measure. 
Even  in  the  study  of  the  laws  of  radiation  from  a  hollow  enclosure  or 
Kirchhoff  radiator  (so-called  "  black-body "),  the  direct  galvanometer 
deflections  were  observed  and  reduced  to  a  single  standard  of  sensitive- 


164  RADIO  METRY. 

ness  which  was  in  arbitrary  units.  The  same  may  be  done  with  the  radi- 
ometer. Its  sensitiveness  is  easier  to  control,  since  it  can  be  made  to 
depend  only  upon  the  pressure  of  the  residual  gas  —  the  constant  of  a 
galvanometer  varies  continually.  It  is  not  affected  by  magnetic  effects, 
and  a  heavy  vane  is  less  affected  by  earth  tremors  than  is  a  very  light 
galvanometer  suspension.  It  is  sensitive  to  temperature  changes,  but  less 
so  than  the  bolometer,  and  it  can  be  more  easily  shielded  from  temperature 
changes  than  can  a  bolometer  with  its  galvanometer,  battery,  etc.  The 
fact  that  it  is  not  portable  is  not  a  serious  drawback,  since  it  is  not  usually 
necessary  to  move  the  instrument.  It  has  two  disadvantages,  viz,  its 
window,  or  preferably  double  window,  is  selective  in  its  transmission,  and 
its  period  is  somewhat  longer  than  that  of  a  bolometer  and  galvanometer 
of  equal  sensitiveness.  But  the  latter  is  nearly  always  drifting  and  to 
repeat  one's  readings  it  takes  as  long  for  an  observation  as  it  does  with  a 
radiometer. 

Since  the  weight  is  of  minor  importance,  tremors  are  avoided  by  having 
the  suspension  weigh  about  8  to  10  mgs.  When  used  with  a  good  mercury- 
pump,  it  requires  no  attention  after  it  is  properly  adjusted.  A  delicate 
galvanometer  requires  frequent  adjustment  and  in  connection  with  a 
bolometer  the  investigator's  time  is  occupied  principally  with  the  care 
of  the  instrument  (at  least  that  has  been  the  writer's  experience),  which 
should  be  a  secondary  matter.  The  two  instruments  are  of  the  same 
order  of  sensitiveness,  with  the  possibility  of  the  radiometer  being  the 
more  sensitive. 

This  is  well  illustrated  in  the  test  for  their  efficiency  to  ultra-violet 
radiation,  where  both  instruments  were  at  about  their  maximum  working 
sensitiveness.  The  bolometer  used  was  0.22  by  10  mm.  in  area,  resistance 
2.8  ohms,  and  for  a  battery  current  of  0.04  amp.  with  a  galvanometer 
sensitiveness  of  z=i.5Xicr10  amp.  (period  16  seconds)  had  a  tempera- 
ture sensitiveness  of  9X10"°  per  millimeter  deflection,  on  a  scale  at  i  m. 
(see  table  VII).  (A  Nernst  heater  was  also  used  in  making  the  com- 
parison.) 

A  candle  at  i  m.  gave  a  deflection  of  45  cm.  which,  on  the  assump- 
tion that  the  sensitiveness  is  proportional  to  the  square  root  of  the  area 
of  bolometer  strip,  is  30  cm.  per  square  millimeter.  For  the  radiometer 
having  a  vane  0.5  by  9  mm.  a  candle  gave  a  deflection  equivalent  to  159  cm. 
at  i  m.  or,  since  the  deflection  is  proportional  to  the  area  of  the  exposed 
vane,  35  cm.  per  square  millimeter  (table  VI).  In  other  words,  the  radi- 
ometer was  1.2  times  as  sensitive  as  the  bolometer,  or  i  mm.  deflection 
corresponded  to  7.5°Xio~8C.  (For  a  full  period  of  2.5  minutes  its  sensi- 
tiveness was  3.8° Xio~6  C.)  Its  period,  however,  was  4.5  times  that  of 
the  bolometer.  This  estimation  of  sensitiveness  is  based  on  the  assump- 
tion that  the  radiometer  was  as  complete  an  absorber  of  energy  as  the 
bolometer.  Judging  from  its  period,  its  efficiency  is  much  lower  than 


THE   BOLOMETER.  165 

that  of  a  bolometer,  hence  the  radiometer  must  be  sensitive  to  temperature 
changes  less  than  the  value  just  given. 

The  sensitiveness  of  bolometers  thus  far  attained  is  about  1  OOQ  000 
degree  per  millimeter  deflection.  Paschen  claims  a  sensitiveness  of 
10  00*0  ooo  degree  by  reading  to  o.i  mm.  But  the  conditions  are  rare 
when  one  can  read  to  o.i  mm.,  so  that  the  estimate  would  seem  too  high. 
As  will  be  seen  presently,  the  working  sensitiveness  is  of  the  order  of 
1  OOQ  000  degree,  or  generally  considerably  less. 

V.     THE   BOLOMETER  WITH   ITS   AUXILIARY   GALVANOMETER. 

We  have  now  to  consider  one  of  the  most  useful  radiation  meters  yet 
devised,  viz,  the  bolometer  which  is  simply  a  Wheatstone  bridge,  two 
arms  of  which  are  made  of  very  thin  blackened  metal  strips  of  high  elec- 
trical resistance  and  high  temperature  coefficient,  one  or  both  of  which 
are  exposed  to  radiation.  When  thus  exposed,  their  temperature  changes, 
thus  unbalancing  the  bridge,  and  the  resulting  deflection  of  the  galvanom- 
eter gives  a  measure  of  the  energy  absorbed.  The  maximum  sensitive- 
ness of  the  bolometer  is  limited  by  the  size  of  the  strip  and  is  proportional 
to  the  square  root  of  the  surface  exposed  to  radiation.  Any  further  gain 
in  sensitiveness  must  be  attained  by  increasing  the  sensitiveness  of  the 
galvanometer,  which,  for  the  moving  magnet  type,  varies  as  the  square  of 
its  (undamped)  period.  The  sensitiveness  is  also  proportional  to  the 
bolometer  current,  which  is  limited  by  the  resistance  of  the  bolometer 
strips. 

It  will  be  noticed  presently  that  the  working  sensitiveness  of  the  vari- 
ous galvanometers  thus  far  used  is  of  the  order  of  2Xio~10  amp.  per 
millimeter  deflection,  while  the  temperature  sensitiveness  varies  from 
5°Xio~5  to  5°Xio~6  for  i  millimeter  deflection  for  a  scale  at  i  meter. 

HISTORICAL. 

The  various  bolometer-galvanometer  apparatus  will  first  be  noticed, 
in  so  far  as  it  relates  to  spectro-radiometric  work. 

The  first  great  step  in  improving  the  moving  magnet  galvanometer  is 
due  to  Kelvin  who  decreased  the  weight  of  the  moving  parts  to  a  few 
milligrams,  and  introduced  the  astatic  system  of  magnets.  The  main 
problem  in  bolometer  construction  is  to  use  strips  of  a  metal  having  a  high 
resistance-temperature  coefficient,  a  small  specific  heat,  and  low  heat  con- 
ductivity. These  metals  are  nickel,  platinum,  tin,  and  iron,  but  for  vari- 
ous reasons  in  mechanical  construction,  platinum  is  the  most  commonly 
used. 

The  manner  in  which  this  instrument  has  been  developed  to  its  pres- 
ent high  sensitiveness  is  best  illustrated  by  considering  the  various  designs 
of  different  investigators,  given  in  table  VII. 


i66 


RADIOMETRY. 
TABLE  VII.— BOLOMETER-GALVANOMETER  SENSITIVENESS. 


Galvanometer. 

Bolometer. 

Observer. 

Resist- 
ance. 

Full 
period. 

Current 
sensitiveness 
for  i  mm. 

Resist- 
ance. 

Area. 

Bolometer 
current. 

deflection. 

Langley  (AnnalsAstrohpys. 

ohms. 

sec. 

I  to  5X  IQ—  ** 

ohms. 
4 

sq.  mm. 
0,05  to  O.O2 

amp. 
o  03 

Obs.) 

XI2 

Abbot  (Astrophys.  J.,  18, 

Pi:  IQOT.  )    . 

i  6 

2O 

5Xio~u 

Angstrom: 
Wied.    Ann.,   26,    p. 

2Sr,  i88c;  

Wied.     Ann.    36,    p. 

715;  1889  

O.IXI2 

Wied.    Ann.,   48,    p. 

407'   l8Q3 

8 

16 

5.7Xio-9 

5 

Julius  (Licht  und  Warme- 

strahlung,     p.     31; 

I&QO) 

2  7 

4Xio~9 

O  7  X  14 

O  T  "?•? 

Helmholtz    (Verb.    Phys. 

Gesellsch.,     Berlin, 

7,  p.  71;  1888).  . 

8Xio~9 

88 

Lummer  and  Kurlbaum: 

Zs.  fur  Instrumenten- 

kunde,   12,    p.    81; 

1892 

i  t;X  io~~9 

60 

I2X  IX  32 

o  006 

Wied.    Ann.,   46,    p. 

2O4"    l802 

Rubens:  Wied.  Ann.,  37,  p. 

- 

255;  1889  

<; 

4  to  f? 

3.2Xio~10 

^.2 

7Xo.^X  3"% 

O.2 

Wied.   Ann.,    45,    p. 

238;  1892  

80 

3.2Xio-10 

3 

3X0.2X10 

BOLOMETERS. 


i67 


TABLE  VII.  —  BOLOMETER-GALVANOMETER  SENSITIVENESS. 


Temperature  sensitiveness. 

Candle  test. 

V 

Remarks. 

For  i  mm.  de- 
flection and 
period  given 
in  col.  3. 

?  Ibid., 
for  a  scale 
at  i  m. 

^Ibid.,  for 
a  scale  at  i  m. 
and  a  full 
period  of 
15  sees. 

Distance  of 
candle. 

3  1  Distance  of 
S.  1  galvanometer. 

•sg 

11 

0-S 

Equivalent  de- 
flection per  mm2 
area  for  candle 
and  scale  at  i  m. 

°C. 
iXio-6 

°C. 

°C. 

met. 

cm. 

In  practice  for  solar  spectrum 
work  a  full  period  of  3  sees, 
and  i  =  2.2Xio~9    ampere  is 
used.  Balancing  coils  of  plati- 
noid. Bolometer  is  inclosed  in 
water  jacket. 
In  practice  drifting  is  minim- 
ized by  placing  a  short  copper 
wire  in  series  with  one  of  the 
bolometer  arms. 
Surface  bolometer;  23  strips  of 
tin  foil;  blackened  with  PtCl2 
and  soot.    Balancing  coils  of 
copper  wire  in  separate  box. 
Sensitiveness    556X10"°    gr- 
cals/cm2/sec. 
Single  Pt.  strip  0.1X12  mm.  in 
spectrobolometer. 
2  arms  of  tin  foil  in  form  of  grat- 
ing.   Sensitiveness  also  given 
as  I27X  io~9  gr-  cals/cm2  sec. 
Bolometer  strips  of  nickel  0.002 
mm.  thick.   Balancing  coils  of 
platinum  wire  in  oil  in  sep- 
arate box.     Found  deflection 
proportional  to  current. 
Surface    bolometer,     4    similar 
arms,  so  no  compensating  re- 
sistances  needed;    diagonally 
opposite  arms  exposed.    Sen- 
sitiveness also  given  as  533  X 
io~9  gr-cals/cm2/sec. 
Surface    bolometer     4    similar 
arms  of  platinum  foil   o.ooi 
mm.  thick;   2  diagonally  op- 
posite arms  exposed  to  radia- 
tion surface  consists  of  1  2  strips 
of  platinum,  i  mm.  wideX32 
mm.   long,   separated   by  1.5 
mm.,  back  of  which  openings 
are   12   similar  strips  of  the 
diagonally  opposite  arm.    Bo- 
lometer currents   as  high  as 
0.04  ampere  could  be  used. 
Surface  bolometer  of  tin  foil  o.oi 
mm.  thick;  7  strips  in  each  arm. 
2  arms  of  0.04  mm.  iron  wire 
hammered    flat;    3    strips    in 
each  arm. 

2 

9Xio~5 

8Xio~5 
3Xio-4 

2X10-* 
5Xio-6 

I 

800 

5 

60 

2.5XIO-5 

4Xio-5 

I 

5 

i68 


RADIOMETRY. 
TABLE  VII  —  CONTINUED. 


Observer. 

Galvanometer. 

Bolometer. 

Resist- 
ance. 

Full 
period. 

Current 

sensitiveness 
for  i  mm. 
deflection. 

Resist 
ance. 

Area. 

Bolometer 
current. 

Rubens:  Wied.  Ann.,  45,  p 
218;  1802  .  . 

ohms. 
80 

15° 
140 

sec. 

• 
3.2XIO-1 

T.SXio-" 
1.5X10-" 

6Xio~u 
6Xio~12 

2.3X10-" 
3.3X10-12 
1.6X10-" 
8.3Xio-12 

2.5X10-!° 
i.5Xio~10 

ohms 
80 

80 

75 

13 

5 

sq.  mm. 
0.09X12 

0.09X12 

0.05  X  7 

amp. 

Rubens  and  Snow  (Wied 
Ann.,  46,    p.    529 
1892) 

2O 
20 

2O 

Snow  (Phys.  Rev.,  i,  p.  31, 
180^    . 

0.025 
0.04 

Aschkinass    (Wied.   Ann., 
55>  P.  4OI>  *895)  .  . 

Donath  (Wied.  Ann.,  58, 
p.  609;  1896)    . 

Paschen  (Wied.  Ann.,  48, 
p.  272;  1893)    .... 

Coblentz    

*6o 
*6o 

20 
5-2 

5-2 

7 
i5 
30 
34 

10 
16 

O.OI  tO  O.O2 

12 

8 

3X0.5X15 

0.25  X7 

0.03 
.02 

2.8 

0.22X10 

O.O2 

Rubens  Thermopile  
Coblentz  Radiometer  .... 

5 

14 
70 

1.4X10-1° 

3-5 

0.8X20 
0.5X9 
0.5X9 

I^O   . 

*Galvanometer;  4  coils  40  mm.  external  and  5  mm.  internal  diameter,  1,200  turns  of  graded  wire  in  each  coil; 
moving  system,  13  magnets  in  each  group,  i  to  1.5  mm.  long,  on  both  sides  of  glass  staff,  and  0.3  mm.  apart; 
mirror  2  mm.  diameter  X  0.03  mm.  thick.  Total  weight  of  system  =  5  mg. 


BOLOMETERS. 


169 


TABLE  VII  —  CONTINUED. 


Temperature  sensitiveness. 

Candle  test. 

*s 

Remarks. 

For  i  mm.  de- 
flection and 
period  given 
in  col.  3. 

Ibid., 
for  a  scale 
at  i  m. 

Ibid.,  for 
a  scale  at  i  m. 
and  a  full 
period  of 
15  sees. 

*o 

Distance  of 
galvanometer. 

ol 

Equivalent  de- 
flection per  mm2 
area  for  candle 
and  scale  at  i  m. 

°C. 
8Xio~6 

3Xio-e 
8Xio~6 

3Xio~5 

°C. 
4Xio-5 

°C. 

met. 
I 

met. 
5 

cm. 
12 

A.O 

2.7 

2  arms  of  0.005  mm-  platinum 
wire  hammered  flat.     Deflec- 
tions  of    galvanometer    pro- 
portional to  current  through 
bolometer  and  rise  in  temper- 
ture  of  arms  proportional  to 
incident  energy. 

For  Hefner  candle  at  i  m. 

2  bolometer  arms  of  platinum 
0.000936  mm.  thick;  balanc- 
ing coils  of  German  silver;  4- 
coil    galvanometer,    total    of 
7200  turns,  wound  with  two 
sizes  of  wire.    Magnet  system 
of    12   magnets  3    to   4   mm. 
long;  total  weight  of  system 
80  mg. 
2  bolometer  arms  of  3  iron  wires 
hammered  flat. 
2  arms  of  platinum  0.17  mm. 
wideX  0.0074  mm.  thick;  bal- 
ancing coils  (of  platinum)  of 
elaborate  design   in   separate 
box.     Drifting  was  serious. 
Bolometer    arms    of    platinum 
o.ooi  to  0.0005  mm.  thick. 
Compensation  resistances  of  man- 
ganin  wire,  final  adjustment 
on  platinum  wire  with  mer- 
cury contact. 
Using  an  8-magnet  system  and  a 
full  period  of   20  sees.  i=8X 
io~n  ampere,  while  a  6-mag- 
net  system  had  a  sensibility  of 
i  =  2.5Xio~u    ampere  for  a 
full  period  of  36  seconds. 

2.4Xio-5 
i.2Xio~4 

4Xio-5 

I 

3 
4 

15 

8-3 

iXio~6 
84  X  io-7 

2.7Xio~6 
2.1  Xio~5 

nXio-6 
8Xio-« 

2  7 

2-5 

9Xio~6 

9Xio~6 

I 

I 

"(iO 

I 
I 

45 

30 

i.iXio-6 

iXio~6 

5 
2-3 

3 

10 

30 
35-5 

15 

35 
7i 

t7-5Xio-« 
J3.8XIO-6 

fThese  values  are  obtained  by  comparing  the  deflections  per  unit  area  with  the  bolometer. 


170 


RADIOMETRY. 

COMPARISON  OF  SENSITIVENESS  OF  VARIOUS  BOLOMETER-GALVANOMETER 

COMBINATIONS. 


The  most  important  data  on  sensitive  radiation  meters,  and  particu- 
larly that  relating  to  bolometers,  is  given  in  table  VII.  It  will  be  noticed 
that  the  thermopile  is  as  sensitive  as  the  bolometer.  The  sensitiveness  of 
the  radiometer  is  obtained  by  comparing  it  with  the  bolometer.  Although 
the  radiometer  is  no  doubt  less  efficient  thaji  the  bolometer,  it  probably 
absorbs  as  much  of  the  incident  energy.  Since  the  radiometer  deflections 
were  larger,  per  unit  area,  than  the  bolometer  deflections,  it  is  safe  to 
assume  that  the  radiometer  was  just  as  sensitive  as,  if  not  more  so  than, 
the  bolometer.  The  long  period  is  of  course  a  serious  objection  in  certain 
classes  of  work.  In  table  VII  it  will  be  noticed  that  the  high  temperature 
sensitiveness  of  the  various  instruments  has  been  attained  by  the  use  of 
a  highly  sensitive,  long  period,  galvanometer,  by  using  a  large  bolometer 
current  and  by  placing  the  scale  at  a  great  distance  from  the  galvanometer. 
Assuming  that  the  sensitiveness  is  proportional  to  the  square  of  the  period 
(undamped)  for  a  scale  at  i  m.  and  a  bolometer  current  of  0.04  ampere, 
it  will  be  noticed  from  column  10,  table  VII,  that  the  temperature  sensitive- 
ness of  the  various  instruments  falls  in  two  groups.  To  the  first  group 
belong  the  earlier  instruments  of  Rubens,  of  Snow,  and  of  Paschen,  with 
a  sensitiveness  of  about  5°X  io~5  per  millimeter  deflection.  To  the  second 
group  belongs  a  more  sensitive  combination  of  Paschen' s,  and  the  writer's 
instrument,  in  which  i  mm.  deflection  corresponds  to  a  rise  in  temperature 
of  n°Xio~6  and  9°Xio~6C.,  respectively.  In  other  words,  the  instru- 
ments of  the  latter  group  have  the  same  sensitiveness,  and  any  increase  in 
the  same  is  to  be  attained  by  increasing  the  scale  distance;  the  bolometer 
current  of  0.04  amp.  is  about  the  limit  for  accuracy.  The  sensitiveness 
of  the  writer's  instruments  could  have  been  further  increased  by  length- 
ening the  scale  distance  to  2.5  m.,  when  the  temperature  sensitiveness 
would  have  been  3.6°Xio~e,  and  by  doubling  the  galvanometer  period, 
when  the  sensitiveness  would  have  been  9°Xio~7  against  Paschen's  i°X 
io~6.  Such  a  computation  is,  of  course,  illusory  on  account  of  damping 
in  the  galvanometer.  On  actual  trial  (but  not  for  the  magnet  system 
quoted)  for  a  full  period  of  30  seconds  the  sensitiveness  of  the  galvan- 
ometer was  i=7Xio~n  ampere. 

COMPARISON  OF  A  BOLOMETER  WITH  A  THERMOPILE. 

The  efficiency  of  the  bolometer  and  the  thermopile  is  reduced  by  losses 
due  to  reflection  and  radiation  from  the  receiving  surface,  and  by  heat 
conduction  to  the  unexposed  parts.  The  loss  of  energy  in  the  thermopile 
due  to  the  Peltier  effect  has  been  considered  in  discussing  that  instrument. 
The  loss  of  energy  due  to  reflection  is  about  4  per  cent  (Kurlbaum,  loc. 
cit.).  Assuming  the  bolometer  to  be  made  of  platinum  0.5X0.002  mm. 
cross-section,  and  the  thermopile  of  20  junctions  of  iron  and  constantan 


THE   BOLOMETER.  171 

wire  0.06  mm.  diameter,  it  can  be  readily  shown  that  the  cross-section  of 
the  thermopile  is  about  56  times  that  of  the  bolometer,  and  from  their 
heat  conductivities,  for  the  same  temperature  gradient,  that  the  loss  of 
heat  by  conduction  in  the  thermopile  is  about  100  times  that  of  the  bo- 
lometer. But  the  temperature  gradient  at  the  ends  of  a  bolometer  strip 
carrying  an  electric  current  may  be  50°  to  100°  C.,  so  that  the  heat  lost  by 
conduction  may  be  about  the  same  for  both  instruments.  Since  the 
temperature  of  the  bolometer  is  from  50°  to  100°  C.  higher  than  the  ther- 
mopile, the  loss  of  heat  per  second  due  to  radiation  in  the  former  is  from 
2  to  3  times  that  of  the  latter.  But  the  mass  of  the  thermopile  is  5  times, 
while  its  specific  heat  is  3.3  times,  that  of  the  bolometer.  Hence,  to  raise 
the  temperature  of  thermopile  and  the  bolometer  to  the  same  extent,  16 
(5X3.3)  times  as  many  heat  units  must  be  supplied  to  the  former.  Since 
the  loss  by  radiation  is  3  times  as  great  from  the  bolometer,  it  will  require 
about  5  times  as  long  for  the  thermopile  to  reach  a  steady  temperature. 
In  practice,  however,  on  account  of  the  blackening  of  the  surface,  the 
bolometer  is  not  so  quick  in  its  action  as  here  computed. 

From  these  considerations,  as  well  as  the  mechanical  difficulties  in 
constructing  a  thermopile  of  wire  less  than  0.05  mm.  in  diameter  (and 
keeping  the  resistance  low) ,  it  will  be  seen  that  the  thermopile  can  not  be 
made  so  quick  in  its  action  as  the  bolometer  and  hence  is  not  so  well 
adapted  where  a  quick  automatic  registration  of  the  galvanometer  deflec- 
tions is  desired.  But,  as  will  be  shown  presently,  since  it  is  difficult  to 
read  large  deflections  accurately  in  less  than  a  4  to  5  second  single  swing 
of  the  galvanometer  system,  a  thermopile  of  0.06  to  0.08  mm.  wire,  which 
attains  a  steady  temperature  in  this  interval  of  time,  is  not  objectionable, 
and,  since  it  is  less  disturbed  by  air-currents  (being  at  room  temperature), 
it  maybe  the  more  reliable  instrument  (see  table  IV).  Neither  instrument, 
however,  compares  with  the  radiometer  in  steadiness.  The  amount  of 
work  done  on  emission,  absorption  and  reflection  spectra,  as  well  as  the 
accuracy  attained,  in  the  infra-red  to  1 5  //,  where  the  radiometer  deflections 
were,  again  and  again,  only  a  few  tenths  of  a  millimeter  would  not  have 
been  possible  with  these  instruments.  In  a  recent  examination  of  reflection 
spectra  of  minerals,  using  a  thermopile,  the  accuracy  attainable  without 
repeating  the  readings  several  times  was  far  from  that  of  the  radiometer, 
although  the  actual  deflections  were  larger. 

The  present  experimental  comparison  of  the  thermopile,  of  0.08  mm. 
iron  and  constantan  wire,  and  the  platinum  bolometer  was  undertaken  in 
order  to  determine  the  accuracy  attainable  in  measuring  a  constant  source 
of  radiant  energy,  and  hence  to  learn  the  feasibility  of  substituting  the 
thermopile  for  the  troublesome  bolometer.  Within  experimental  error  it 
has  been  established  by  Langley,  by  Rubens  and  by  Julius  that  the  bolom- 
eter (galvanometer)  deflections  are  proportional  to  the  current  flowing 
through  the  bolometer,  and  also  to  the  amount  of  energy  falling  upon  the 


172  RADIOMETRY. 

bolometer  strip.  It  remains,  therefore,  to  determine  whether  the  present 
bolometer  behaves  likewise,  and  also  whether  the  same  accuracy  is  attain- 
able with  the  thermopile. 

To  this  end  a  bolometer  was  constructed  with  the  greatest  care.  It 
was  annealed  before  adjusting  the  resistance  of  the  strips,  covered  elec- 
trolytically  with  platinum  black  (after  the  method  of  Kurlbaum)  and  then 
smoked  over  wire  gauze  over  a  paraffin  candle.  The  resistances  of  the 
bolometer  strips  were  1.782  and  1.797  (A  =  0.015)  ohms,  respectively. 
After  blackening  them  they  were  1.766  and  1.818  (A  =  0.052)  ohms,  re- 
spectively. The  width  of  the  bright  strips  was  0.5  mm.,  which  increased 
to  0.56  mm.  after  blacking.  The  length  was  n  mm.  and  thickness  less 
than  0.002  mm.  The  bolometer  current  was  0.04  ampere  and  throughout 
the  following  experiments  there  was  no  difficulty  due  to  air-currents,  or 
drift.  Magnetic  disturbances  were  at  a  minimum,  and,  as  a  whole,  condi- 
tions for  accurate  measurements  were  as  perfect  as  one  would  expect. 

The  thermopile  of  0.08  mm.  wire  (20  junctions  covered  with  a  slit 
0.5  mm.  wide)  already  described,  showed  a  slight  lag  in  registering  the 
energy  received.  Although  this  was  not  marked,  there  was  a  tendency 
for  the  deflection  to  creep,  instead  of  stopping  abruptly  as  in  the  case  of 
the  bolometer.  This  was  most  marked  in  large  deflections,  and  necessi- 
tated exposing  the  thermopile  to  radiation  for  a  definite  time  (6  seconds) 
and  taking  the  zero  at  the  expiration  of  an  equal  interval  of  time.  The 
results  are  given  in  table  VIII.  The  last  two  values  for  the  thermopile  are 
vitiated  by  radiation  from  the  rotating  disk,  to  be  noticed  presently.  The 
results  show  that  there  is  no  great  difference  in  the  two  instruments.  It 
was  necessary,  however,  to  note  the  time  of  exposure  of  the  thermopile, 
which  is  not  convenient  for  large  deflections.  The  estimation  of  the 
relative  merits  of  the  bolometer  and  the  thermopile  is,  therefore,  a  personal 
one,  and  from  the  experience  gained  it  may  be  said  that  for  measuring  in- 
tense sources  the  bolometer  is  the  more  accurate  (when  working  to  0.5  per 
cent)  unless  great  precautions  be  taken  in  making  the  thermopile  readings. 
The  theoretical  temperature  sensitiveness  of  the  thermopile  was  con- 
siderably greater  than  that  of  the  bolometer,  as  was  found  on  subsequent 
computation.  It  may  be  added,  therefore,  that  if  the  bolometer  sensitive- 
ness had  been  increased,  by  increasing  the  current  through  it,  there  would 
have  been  greater  unsteadiness  in  the  galvanometer  readings. 

EXPERIMENT  WITH  A  SECTORED  DISK. 

In  comparing  the  relative  merits  of  the  bolometer  and  the  thermopile, 
the  simplest  method  appeared  to  be  to  reduce  the  intensity  of  the  source 
by  a  known  amount,  by  using  a  sectored  disk,  the  angular  openings  of 
which  are  accurately  known.  It  will  be  noticed  that  while  the  ratios  of 
energy  transmitted  by  the  sectored  disk  were  in  close  agreement  in  any 
series  of  measurements  (see  tables  VIII  and  IX)  the  numerical  values  were 


THE    BOLOMETER.  173 

in  all  cases  higher  than  the  true  ones.  In  other  words,  the  disk  transmitted 
too  much  energy,  or  the  apparent  opening  was  larger  than  the  true  one. 
It  remained,  therefore,  to  be  shown  whether  this  was  due  to  diffraction  (of 
the  very  long  wave-lengths)  or  to  lack  of  proportionality  in  the  registering 
of  the  energy  by  the  bolometer  and  the  thermopile.  The  method  of  ob- 
servation consisted  in  taking  from  5  to  10  readings  without  the  disk,  then 
a  similar  number  with  the  rotating  disk  interposed,  followed  by  a  number 
without  the  disk. 

TABLE  VIII.  —  COMPARISON  OF  BOLOMETER  AND  THERMOPILE. 


Direct  deflection 
(mean  value). 

Deflection  with 
disk,  i8o°=49.g 
(mean  value). 

Ratio. 

Direct  deflection 
(mean  value). 

Deflection  with 
disk,  i8o°  =  49.9 
(mean  value). 

Ratio. 

Bolometer: 

Thermopile: 

May  24,  '07: 

12.22 
12.  l6 

6.14 
6.ii 

*50.24 
*50.24 

May  24,  '07: 
iS-U 
14.22 
14.30 

6.61 
7.21 
7.24 

t50.36 

tS°-7 
J50.6 

*  Nernst  heater  on  98  volts  at  i  m.  from  bolometer.  Galvanometer  (full)  period  8  seconds  undamped;  50  ohms 
in  series.  Total  deflection  about  80  cm.  Bolometer  is  perfectly  steady  and  comes  to  rest  abruptly.  Readings 
vary  from  o.i  to  0.7  per  cent  from  mean  of  about  10  in  each  set.  Temperature  sensitiveness  =6° X  io~ 5. 

t  Conditions  same  as  for  bolometer.  Thermopile  deflection  "creeps,"  and  does  not  come  to  rest  in  same  time 
as  galvanometer  (on  open  circuit  or  with  bolometer),  due  to  its  larger  heat  capacity.  Galvanometer  single  swing 
of  4  seconds  increased  to  6  seconds  and  is  fully  damped,  due  to  lag  of  thermopile;  50  ohms  in  series  with  galvanom- 
eter. Temperature  sensitiveness  =  2°  X  io~6. 

J  Source  and  disk  nearer  screen.  Difficult  to  read  deflection  on  account  of  "creeping,"  which  amounts  to  i  to 
3  mm.  Readings  made  at  end  of  6  seconds  vary  by  0.4  per  cent  from  mean;  20  ohms  in  series  with  the  galvanom- 
eter. Temperature  sensitiveness  =  s°X  lo""6. 

The  first  test  made  was  to  determine  whether  the  rotating  disk  (30  cm. 
diameter,  1.3  m.  from  the  bolometer)  affected  the  instrument;  and  it  was 
found  that  the  resulting  deflections  i  to  2  mm.  were  no  larger  than  those 
due  to  stray  radiation  reflected  from  the  stationary  disk.  A  heavy  black 
cardboard  shield  was  then  placed  between  the  bolometer  and  the  disk 
(0.5  m.  from  the  disk)  and  similar  screens  were  placed  around  the  source, 
which  was  2  m.  from  the  bolometer.  No  radiation  was  detected  from  the 
stationary  disk,  whether  the  open  or  closed  part  of  the  disk  faced  the 
bolometer;  but  unfortunately  this  test  was  not  made  for  the  moving  disk. 
The  disk  with  the  240°  opening  gave  values  0.5  per  cent  too  high  (see 
table  IX).  The  results  with  the  120°  disk  (6  openings  of  20°  each)  were 
in  still  greater  error.  The  space  between  the  bolometer  and  the  shield 
was  then  entirely  inclosed,  and  with  the  disk  close  to  the  opening  (7  X 10 
cm.)  in  the  shield  the  discrepancy  became  still  greater.  It  was  then  found 
that  the  increased  transmission  is  due  to  the  moving  disk  and  depended 
upon  the  distance  of  the  disk  from  the  screen. 

It  was  further  shown  that  the  transmission  was  proportional  to  the 
speed,  so  that  the  240°  disk  (true  transmission  66.827  per  cent1)  gave 


1  These  disks  and  their  constants  were  supplied  by  Dr.  Hyde.  ^  Bureau  of  Standards, 
Bulletin,  2,  p.   i,   1906. 


174 


RADIOMETRY. 


values  from  69.3  to  77.2  per  cent.  In  fig.  105  are  plotted  the  galva- 
nometer deflections  for  different  distances  of  the  disk  (abscissae)  from  the 
screen.  The  latter  was  80  cm.  (source  at  2  m.)  from  the  bolometer,  and 
had  an  opening  in  it  which  was  the  size  of  the  openings  in  the  sectored  disk. 
No  radiation  was  observed  from  the  stationary  disk,  nor  from  the  shields 
back  of  it  when  the  open  sector  was  before  the  bolometer.  The  speed  of 
the  disk  was  such  as  is  used  in  photometry,  and  was  kept  constant  for 
each  series  of  observations.  In  the  lower  curve  for  the  240°  disk  the 
speed  was  slow  and  there  was  a  flicker  on  viewing  it.  The  curves  show 
that  the  maximum  radiation  occurs  when  the  disk  is  about  6  cm.  from 
the  shield;  and  it  disappears  immediately  on  stopping  the  disk. 

TABLE  IX.  —  RELIABILITY  OF  BOLOMETER  MEASUREMENTS. 


Direct  deflection,  a 

Deflection  with 
disk,  a 

Ratio. 

Direct  deflection,  a 

Deflection  with 
disk,  a 

Ratio. 

Disk  opening,  240°=  66.827. 

Disk  opening,  120°  =33.406. 

9.42  cm. 

9-35 
11.82 
9.11 
8.94 
I3-3I 

6.33  cm. 

6.28 
7.95 

6.12 

6.04 

8.99 

6  67.23 
b  67.25 

667.25 
667.2 
'67.5 

667.6 

15.82 
11.56 
12.31 
30.40 
13-75 
I4-I5 
17.04 

5-65 
4.14 
4.28 

iQ-53 
4.70 

4-75 
5.68 

«35-7° 
/35-7S 
£34-8 
^34-7 
*34-5 
33-56 
33-38 

Disk  opening,  180°  =50.125. 

17.17  cm. 
17.21 

8.65 

8.64 

*  5°-37 
d  50.  20 

a  Mean  of  6  to  10  readings. 

b  Galvanometer  period  8  seconds,  vibration  undamped;  20  ohms  in  series  with  galvanometer.  Temperature 
sensitiveness  =  6° X  io~5  C.  High  values  due  to  radiation  from  moving  disk,  which  is  0.5  m.  from  screen. 

c  Galvanometer  period  14  seconds  and  vibration  just  damped.    Temperature  sensitiveness  =  i°  .2X 10— 5  C. 

d  Nernst  heater  on  98  volts  at  i  m.  from  bolometer.  Galvanometer  period  8  seconds.  30  ohms  in  series  with 
galvanometer.  Total  deflection  about  80  cm.  Individual  deflections  vary  0.2  to  0.5  per  cent  from  mean. 

e  Galvanometer  period  14  seconds. 

/  Galvanometer  period  8  seconds.  Disk  better  shielded  than  in  previous  experiments  and  closer  to  screen  — 
6  cm.  from  it.  Heater  150  cm.  from  bolometer;  screen  at  80  cm. 

g  14  ohms  in  galvanometer  circuit.    Galvanometer  period  14  seconds.    Nernst  heater  on  74  volts. 

h  No  resistance  in  galvanometer  circuit;  results  show  that  high  value  is  not  due  to  lack  of  proportionality  of 
galvanometer  deflections. 

*  Nernst  heater  on  95  volts.    20  ohms  in  galvanometer  circuit.    Total  deflection  about  45  cm. 

The  motor  was  run  continuously  for  a  complete  series  of  measurements 
and  no  deflections  greater  than  i  to  2  mm.  were  observed  from  it  or  the 
disk,  immediately  after  stopping  the  rotation.  In  these  tests  the  motor 
was  shielded  from  the  bolometer.  On  removing  the  shield  and  the  disk 
and  on  running  the  motor  the  deflections  were  from  i  to  3  mm.  The 
whole  shows  that  the  sectored  disk  is  not  as  applicable  as  one  would  sup- 
pose, unless  one  determines  the  corrections  which  in  two  series  of  experi- 
ments were  found  to  be  in  very  close  agreement.1  In  the  present  curves 

1  The  sector  was  rotated  at  a  higher  speed  than  actually  required  with  a  bolometer.  By 
means  of  suitable  pulleys  the  speed  may  be  reduced  to  perhaps  -fo  that  used  in  the  present 
test,  which  would  decrease  the  errors. 


SUMMARY. 


175 


the  galvanometer  was  at  its  full  sensitiveness  —  no  resistance  in  series  — 
so  that  in  the  actual  experiments  (table  IX)  the  error  was  less  exaggerated. 
For  the  240°  disk  where  the  error  in  the  deflections  was  only  i  or  2  mm. 
the  correction  reduces  the  observed  values  close  to  the  true  one. 

From  the  consistency  of  the  ratios  in  each  series,  which  is  of  the  order 
of  0.2  to  0.3  per  cent,  it  will  be  noticed  that  the  bolometer  is  a  very  reliable 
instrument  in  spite  of  its  mechanical  weakness. 


6          Q         10         12,         14-       /6         18        -20       22        24      36C777 
FIG.  105.  —  Radiation  due  to  rotating  sectored  disk. 

VI.  SUMMARY. 

The  present  paper  deals  with  four  instruments  for  measuring  radiant 
energy,  viz,  the  radiomicrometer,  the  linear  thermopile,  the  radiometer, 
and  the  bolometer  with  its  auxiliary  galvanometer. 

As  a  result  of  this  historical  inquiry  and  by  experiment  it  was  shown 
that  the  radiomicrometer  is  capable  of  great  improvement,  by  reducing 
its  weight,  by  lengthening  its  period,  and  by  placing  it  in  a  vacuum.  It 
was  further  shown  that  on  account  of  para-  and  dia-magnetism  the  sensi- 
tiveness of  the  radiomicrometer  is  very  limited,  perhaps  only  a  fifth  of  the 
best  bolometers  described. 

It  was  also  shown  that  the  Rubens  thermopile  is  as  sensitive  as  the 
best  bolometer,  and  that  its  heat  capacity  can  be  greatly  reduced  by  using 
thinner  (0.06  to  0.08  mm.  diameter)  wires,  which  are  made  shorter,  thus 
keeping  the  resistance  low.  The  computed  errors  due  to  the  Peltier  effect 
are  about  i  part  in  300.  The  thermopile  is  not  so  well  adapted  as  is  the 
bolometer  for  instantaneous  registration  of  radiant  energy  and  it  does  not 
admit  so  great  a  range  in  variation  of  sensibility;  but,  on  account  of  its 
greater  steadiness,  it  commends  itself  for  measuring  very  weak  sources  of 
radiation,  e.g.,  the  extreme  ultra-violet  and  infra-red  region  of  the  spectrum. 

By  a  direct  comparison  it  was  shown  that  the  radiometer  can  be  made 
just  as  sensitive  as  the  bolometer,  but  its  period  will  be  much  longer,  It 


1 76  RADIOMETRY. 

was  found  that  the  radiometer  is  not  selective  in  its  action,  and,  hence, 
that  it  can  be  used  for  measuring  ultra-violet  radiation.  The  main  objec- 
tion to  the  use  of  a  radiometer  is  its  long  period;  but,  since  it  is  easily 
shielded  from  temperature  changes,  and  since  it  is  not  subject  to  magnetic 
perturbations,  this  long  period  is  of  minor  importance  so  long  as  we  are 
dealing  with  a  constant  source  of  radiation.  In  spectrum  energy  work  its 
usefulness  is  limited  to  the  region  in  which  the  window  is  transparent  — 
to  20  IJL  and  from  40  to  60  p  by  using  quartz.  The  fact  that  the  deflections 
of  the*  radiometer  can  not  be  obtained  in  absolute  measure  is  a  minor 
objection,  since  in  but  few  cases  (thus  far  at  least)  has  it  been  necessary 
to  thus  obtain  the  deflections.  The  action  of  a  radiometer  is  somewhat 
analogous  to  a  photographic  plate,  in  that  it  will  detect  weak  radiation, 
provided  one  can  wait  for  it,  and,  on  account  of  its  great  steadiness,  is, 
of  all  the  instruments  considered,  probably  the  best  adapted  in  searching 
for  infra-red  fluorescence. 

A  bolometer  installation  is  so  distributed  that  it  is  difficult  to  shield 
from  temperature  changes.  In  spite  of  its  small  heat  capacity  the  bo- 
lometer has  a  "drift"  due  to  a  slow  and  unequal  warming  of  the  strips. 
Air-currents  which  result  from  the  hot  bolometer  strips  also  cause  a  varia- 
tion in  the  deflections  of  the  auxiliary  galvanometer.  Nevertheless,  despite 
these  defects  it  is  the  quickest  acting  of  the  four  instruments  considered, 
and  is  the  best  adapted  for  registering  the  energy  radiated  from  a  rapidly 
changing  source.  For  precision  work  it  is  necessary  to  keep  the  bolometer 
balanced  to  less  than  i  cm.  deflection. 

The  auxiliary  galvanometer  is  the  main  source  of  weakness  in  measur- 
ing radiant  energy,  and  in  places  subject  to  great  magnetic  perturbations 
a  period  greater  than  5  seconds,  single  swing  is  to  be  avoided.  Hence, 
although  a  greater  sensitiveness  is  possible,  the  working  sensibility  of  the 
various  galvanometers  studied  is  of  the  order  of  i=2Xio~10  ampere  per 
millimeter  deflection  on  a  scale  at  i  m.  Under  these  conditions  the  various 
bolometers  used  were  (as  a  fair  estimate  of  the  recorded  data)  sensitive  to 
a  temperature  difference  of  4°Xio~5  to  5°Xio~e  per  mm.  deflection,  on  a 
scale  of  i  m.  The  galvanometer  sensibility  was  found  to  be  closely  pro- 
portional to  the  period. 

A  direct  comparison  of  the  thermopile  and  the  bolometer  shows  that 
there  is  little  preference,  other  than  a  personal  one,  in  these  two  instru- 
ments. 

The  use  of  a  rotating  sectored  disk  for  reducing  the  intensity  of  the 
source  is  liable  to  introduce  errors,  which  must  be  taken  into  account. 


APPENDIX  III. 

ADDITIONAL    DATA   ON    SELECTIVE    REFLECTION    AS    A    FUNCTION 
OF   THE   ATOMIC   WEIGHT    OF   THE    BASE. 

As  this  work  goes  to  press  the  experiments  of  Morse1  have  been  pub- 
lished, and  since  it  contains  considerable  new  data  for  the  region  of  the 
spectrum  from  10  to  15  //,  it  is  included  here  for  the  sake  of  completeness. 

In  that  paper  considerable  comment  is  made  upon  the  fact  that  the 
writer  (see  Carnegie  Publication  No.  65)  missed  the  reflection  bands  in 
calcite  and  in  magnesite,  previously  found  by  Aschkinass  at  1 1  to  12/1. 
In  reply  it  may  be  stated  that  these  two  substances  were  examined  in  the 
preliminary  work  on  reflection  spectra,  in  order  to  get  a  check  on  the 
calibration;  and,  on  finding  the  reflected  energy  very  weak,  no  attempt 
was  made  to  locate  the  bands  known  to  be  at  1 1  to  12  /*.  In  this  work 
a  Rubens  thermopile  (heavy  wires)  was  used,  which  was  sluggish  and  was 
disturbed  by  air-currents.  Although  the  sensibility  was  higher  than  in 
the  radiometer  previously  used,  the  small  deflections  were  not  so  reliable 
and  no  attempt  was  made  to  locate  weak  reflection  bands  beyond  n  /*, 
such  as  are  found  in  the  carbonates.  This  demonstrates  the  superiority 
of  the  radiometer  for  measuring  weak  radiation. 

The  investigation  of  weak  reflection  spectra  in  the  extreme  infra-red  is 
accomplished  under  great  difficulties,  and  Morse  has  done  an  excellent 
service  in  obtaining  data  in  this  region  of  the  spectrum.  He  used  a  35  cm. 
focal  length  mirror  spectrometer  as  compared  with  the  writer's  52  cm. 
focal  length  mirrors.  In  the  larger  spectrometer  the  energy  in  the  spec- 
trum is  much  weaker,  while  the  resolution  is  greater.  The  shorter  focus 
does  not  militate  against  the  results,  however,  which  show  that  the  simple 
atomic  weight  relation  among  the  carbonates  found  by  the  writer  at  6  to 
8  jj.  holds  for  the  long  wave-lengths  at  n  to  15  /£,  where  the  dispersion  is 
considerably  greater.  The  writer  found  the  reflection  band  of  the  carbon- 
ates at  6  fj.  very  complex  (see  Chapter  III)  and  it  would  be  interesting 
to  learn  whether  the  bands  at  11.4  to  15  /JL  are  likewise.  In  table  X  are 
given  the  maxima  of  the  reflection  bands  of  the  carbonates  examined  by 
Morse.  It  contains  one  new  substance,  MnCO3,  not  examined  by  the 
writer.  The  values  of  the  maxima  of  the  first  band  are  not  always  in 
agreement,  but  this  appears  to  be  due  to  the  difference  in  resolving  power 
of  the  two  instruments.  In  fig.  43  are  plotted  the  wave-lengths  of  the 

1  L.  B.  Morse:  Astrophys.  Jour.,  26,  p.  225,  1907. 

177 


i78 


SELECTIVE  REFLECTION   BANDS. 


reflection  maxima  of  the  bands  at  11.5  and  14.5^  against  the  atomic 
weight  of  the  base.  From  this  it  will  be  seen  that  the  rate  of  shift  of  the 
band  with  increase  in  atomic  weight  of  the  base  is  greater  for  the  bands  at 
11.5  and  14.5  {j.  than  for  those  at  6.6  and  7.0  /*,  just  as  was  found  for  the 
bands  of  the  sulphates  at  4.6  /*,  at  6.2  to  6.6  //,  and  at  8.2  to  9.3  JJL  (see 
figs.  44  and  45). 


Substance. 

Chemical 
composition. 

Atomic 
weight  of 
base. 

Reflection  maxima. 

Band  i. 

Band  2. 

Band  3. 

Magnesite 

MgCOa 
CaCOa 
CaCOa 
MnCO3 
FeCOa 
ZnCOa 
SrC03 
BaCOa 
PbCOa 

24.2 

39-7 
39-7 
54-6 

55-5 
64.9 
86.9 
136.4 
205.4 

6.5  ft 
6.6 
6.65 
6.63 
6.60 
6.7 
6.76 
6.86 
7.2 

II.2/A 

11.31 

"•55 
11.47  + 

11.47- 
11.38 
11.56 
1  1.  60 
11.94 

"S-0*1 

14.2 
14.2 
14.0— 
13-9- 
13-6 
14-37 
14-5 
14-8 

Calcite  

Aragonite     .... 

Rhodochrosite     .        .    . 

Siderite  

Smithsonite    

Strontianite    

Witherite    

Cemssite  

By  arbitrarily  selecting  KNO3  and  AgNO3  from  the  nitrates,  Morse 
found  the  line  drawn  through  the  maxima  of  the  reflection  bands  was 
"approximately  parallel"  to  those  of  the  carbonates  (see  fig.  43).  More- 
over, the  line  drawn  through  the  maxima  of  the  sulphates,  at  8.6  to  9  //, 
is  also  closely  parallel  with  those  of  the  carbonates.  From  this  he  was 
led  to  suspect  that  the  shift  of  the  band  with  increase  in  the  atomic  weight 
of  the  base  is  of  the  same  order  of  magnitude  in  carbonates,  nitrates,  and 
sulphates.  If  the  data  had  been  plotted  to  a  larger  scale,  as  the  accuracy 
of  the  values  of  the  maxima  seem  to  permit,  then  the  lines  would  not  even 
be  approximately  parallel,  as  will  be  noticed  in  figs.  43  and  44. 

After  examining  the  reflection  and  transmission  spectra,  one  or  both, 
of  over  300  substances,  the  observations  lying  in  the  region  of  the  spectrum 
from  0.5  to  30+^,  I  have  found  that  it  is  an  easy  matter  to  work  out  all 
sorts  of  fantastic  relations,  only  to  learn,  after  gathering  more  data,  that 
the  whole  thing  was  an  illusion.  For  this  reason  it  seems  to  me  that  the 
linear  relation  between  the  weight  of  the  element,  combined  with  equal 
amounts  of  oxygen  in  the  acid  radical  found  by  Morse  by  arbitrarily 
selecting  maxima  of  reflection  bands,  is  misleading.  From  the  earliest 
work  of  Abney  and  Festing  to  the  latest  (theoretical)  work  of  Einstein,  it 
has  been  generally  accepted  that  oxygen  is  the  active  element  in  causing 
(at  least  in  "  sharpening  ")  certain  bands,  just  as  sulphur  has  been  found 
quite  inactive.  The  great  groups  of  chemically  related  compounds  have 
been  found  to  have  similar  absorption  and  reflection  spectra,  but  no  sim- 
ple relation  could  be  established  between  the  spectra  of  the  groups  of 
compounds.  The  present  simple  relation  results  from  selecting  particular 
reflection  bands  found  in  certain  carbonates,  nitrates,  sulphates,  and  sili- 


THE   ATOMIC-WEIGHT   LAW. 


179 


cates.  Hence,  the  selection  of  KNO3  (max.  at  7.1  fi)  from  the  nitrates  by 
Morse  seems  arbitrary,  for  Pfund  (loc.  cit.,  see  Carnegie  Publication  No. 
65)  has  given  a  large  number  of  nitrates  which  have  a  band  in  common  at 
7.45/1.  This  seems  to  be  a  characteristic  band  of  trie  nitrates,  and  AgNO3 
might  have  been  selected  instead  of  KNO3.  A  characteristic  band  of  the 
sulphates  appears  to  be  at  9.1  /*  (harmonic  with  the  one  at  4.55  /x),  while 
Morse  selected  a  less  frequent  one  at  8.6  //.  From  the  silicates,  MgSiO3, 
with  an  insignificant  band  at  9.1  /*,  was  selected,  while  N^SiOg,  with  a 
sharper  band  at  9.9  p,  and  Zn2SiO4,  with  a  group  of  still  more  intense 
bands  at  10.1,  10.6,  and  n  fi,  respectively,  were  not  considered. 

In  PbMO4  the  maxima  lie  in  the  region  of  11.75  and  13  //,  while  in 
CaWO4  there  is  a  large  reflection  band  with  maxima  at  about  11.4,  11.9, 
and  12.5  /£,  respectively.  These  data  including  Morse's  are  plotted  in 
fig.  1 06  and  tabulated  in  table  XI,  from  which  it  will  be  observed  that 

TABLE  XI. 


Substance. 

Chemical 
formula. 

Atomic  weight 
of  base. 

Weight  with 
48  gr.  of  O. 

Position 
of  band. 

Calcite 

CaCO3 

•JQ  7 

1  2  gr  of  C 

66  /A 

Potassium  Nitrate 

KNO3 

38  o 

14  cr  of  N 

7  It  ("7  oO 

Anhydrite    

CaSO4 

•JQ  7 

24  2r    of  S 

86 

Enstatite    

MgSiO3 

24.2 

28  gr.  of  Si 

O.I 

Zircon  

ZrSiO* 

QO.6 

28  gr.  of  Si 

10,  10.6,  ii  /* 

Wulfenite  

PbMoO4 

2O6.O 

7  2  gr.  of  M 

ii.  7?,  i"? 

Scheelite 

CaWO4 

•JQ  7 

1  18  ffr  of  W 

114    IIO    12  ^ 

the  graph  is  not  a  straight  line,  and  that  if  any  relation  exists  it  is  a  very 
complex  curve,  showing  that  for  the  region  up  to  10  jj.  the  atomic  weight 
of  the  element  in  the  acid  radical  has  a  great  effect  in  shifting  the  maxi- 
mum, while  beyond  this  point  the  atomic  weight  of  the  element  united 
with  oxygen  is  of  minor  importance.  In  fact,  the  base  and  the  element 
united  with  the  oxygen  in  the  acid  radical  seem  to  influence  each  other. 
As  an  illustration  of  the  arbitrariness  in  selecting  bands  to  establish 
relations  like  the  aforesaid,  calcite  (CaCO3)  may  be  noticed,  in  which  the 
reflection  band  is  complex  with  maxima  at  6.4  (?),  6.5,  6.6,  and  7  /*,  while 
in  its  isomer,  aragonite,  the  maxima  are  at  6.4  (?),  6.52,  6.74,  and  7  (?)  fi. 
In  SrCO3  the  band  with  a  maximum  at  6.67  //  could  not  be  resolved  even 
with  a  fluorite  prism.  Hence,  one  is  at  a  loss  to  know  which  band  (at 
6  to  7  /*)  to  select  from  the  carbonates  to  compare  with  the  sulphates, 
nitrates,  and  silicates.  On  the  other  hand,  in  the  carbonates  and  in  the 
sulphates  the  maxima  of  the  bands  have  been  plotted  in  their  order  of 
occurrence,  which  would  seem  to  eliminate  personal  bias  in  the  selection 
of  maxima.  From  this  it  would  appear  that  the  simple,  linear  relation 
between  the  atomic  weight  of  the  base  and  the  maxima  of  the  reflection 


i8o 


SELECTIVE   REFLECTION   BANDS. 


bands  in  the  carbonates  and  in  the  sulphates  is  in  agreement,  at  least  as 
a  first  approximation.  Even  with  these  data  at  hand,  speculation  in  regard 
to  dynamical  relations  among  atoms  in  the  molecules  had  better  be  post- 
poned until  more  data  have  been  procured. 


Weight  combined  with  03 

f\j^osCoo^j5s\c 
K  o5oo8oooc 

• 

X        X 

x  Cfy^ 

W04 

i 

9 

| 

C 

:? 

X 

PbMo 

04 

O  ^ 
85  "f 

1  '1 

\g& 

^ 

f  f|l 

H 

A  x^    ! 

^n[Si04 

^r 

5           6            7           8           9           10          II           12        /3/J. 

FIG.  106.  —  Maxima  of  reflection  bands. 

The  data  on  the  reflection  bands  of  the  carbonates  at  n  to  15  //  is  of 
interest  in  connection  with  the  question  of  the  value  of  the  extinction 
coefficient  necessary  to  give  rise  to  selective  reflection.  In  Carnegie  Pub- 
lication No.  65,  page  70,  is  given  the  transmission  band  of  a  thin  section 
of  calcite,  from  which  it  will  be  observed  that  the  absorption  band  at  11.4  /* 
coincides  with  the  band  found  by  reflection.  It  would  seem  desirable  to 
examine  the  transmission  of  thin  sections  of  carbonates  in  this  region  of 
the  spectrum,  using  polarized  energy,  to  compare  with  the  intensity  of 
the  bands  found  by  reflection.  The  reflection  bands  at  14  /j.  were  fre- 
quently found  to  be  extremely  weak,  and  in  view  of  the  importance  of  the 
bearing  of  the  results  upon  the  whole  subject,  it  seems  highly  desirable  to 
examine  the  transmission  spectra  of  thin  sections  of  these  minerals  (using 
preferably  polarized  energy),  to  verify  the  aforesaid  observations.  (See 
foot  note  on  page  30.) 


ADDENDUM. 


Mr.  Morse  has  recently  presented  "  Additional  observations  on  the 
selective  reflection  of  salts  of  oxygen  acids"  (Washington  meeting,  Amer- 
ican Physical  Society,  April,  1908).  He  chose  substances  in  which  the 
"  weight  of  the  acid-forming  element  is  not  greater  than  the  weight  of  the 
oxygen  with  which  it  is  combined."  These  substances  are:  CaCO3  with 
a  reflection  maximum  at  6.6  /*;  KNO3  at  7.1  /*;  AgNO3  at  8  /*;  CaSO4  at 
8.6  fi.-  KC1O4  at  9  /*;  CaSiO3  at  9.2  /*;  KC1O3  at  9.9  /*;  KMnO4  at  10.9  /*; 
PbCrO4  at  11.5  /*;  and  CaTiO3  at  14.2  //.  These  maxima  lie  close  to  the 
line  drawn  through  them,  when  plotted  against  the  weight  of  the  oxygen. 
He  excluded  PbMO4,  CaWO4,  etc.,  because  the  weight  of  the  acid-form- 
ing element  is  greater  than  that  of  the  oxygen  present.  Of  course,  if  we 
admit  the  validity  of  such  a  procedure  the  rule  is  proven;  but  a  rule  loaded 
down  with  exceptions  can  not  prove  satisfactory,  and  much  as  all  spectro- 
scopists  wish  to  establish  such  a  simple  relation  between  spectra  of  different 
groups  of  compounds,  it  should  be  along  lines  of  less  arbitrary  elimination. 

On  the  other  hand,  this  simple  atomic-weight  relation  seems  to  hold 
for  substances  belonging  to  the  same 
group  of  compounds,  even  for  re- 
mote parts  of  the  infra-red  spec- 
trum, as  was  shown  by  Nichols 
and  Day  (Washington  meeting, 
American  Physical  Society,  April, 
1908).  They  found  a  band  of 
residual  rays  in  SrCO3  at  43.2  /* 
and  in  BaCO3  at  46.5  /*,  for  the 
carbonates  (see  fig.  45). 

If  we  take  the  ratio  of  the  in- 


60° 


400 


200 


10 


20        30 


40 


SOJLL 


FlG'  I07'  ~Mean  vaMaa  of  •roup8  of  residual  rays* 
crease  in  atomic  weight  of  the  base  to  the  position  of  the  maximum,  as 
read  from  the  graphs  in  figs.  43  to  45,  the  value  for  the  group  of  bands 
at  6.5  /£  is  about  500  to  i,  at  11.5  /JL  it  is  260  to  i,  and  at  45  /*  it  is  20  to  i. 
The  graph  (fig.  107)  of  these  values  appears  to  be  an  hyperbola.  If  such 
a  relation  really  exists  and  if  the  carbonates  have  a  band  in  the  region  of 
30  fj.,  corresponding  to  the  CaCO3  and  MgCO3  bands,  then  the  aforesaid 
ratio  of  atomic  weight  to  position  of  the  maximum  is  about  90  to  i,  and 
one  would  expect  to  find  a  maximum  for  PbCO3  at  31.5  /*  and  SrCO3 
at  30  JJL.  However,  from  the  variation  in  complexity  of  the  bands  at  6  to 
7  p  and  in  intensity  of  the  bands  at  n  to  14  //,  it  is  evident  that  it  is  use- 

181 


1 82  ADDENDUM. 

less  to  attempt  to  predict  their  behavior  in  the  remote  infra-red.  On  the 
whole,  the  present  data  indicate  that  the  greater  the  wave-length  of  the 
maximum  the  greater  the  shift  of  that  maximum  with  increase  in  atomic 
weight  of  the  base. 

The  latest  conclusion  (Morse,  Phys.  Rev.  26,  p.  526,  1908)  is  that 
"  present  data  do  not  form  any  reasonable  basis  for  assuming  that  all  the 
reflection  bands  in  even  the  simpler  oxygen  acfds  are  connected  with  each 
other  by  such  a  simple  relation  as  that  found  to  hold  roughly  for  the  first 
bands,  and  which  may  be  found  to  hold  also  for  the  second  bands  in  the 
salts  examined." 


INDEX   OF   SUBSTANCES. 


PAGE. 

Adularia 98 

Albite    97 

Aluminum    72 

Aluminum  oxide 107 

Ammonium  alum    50 

Ammonium-iron  alum   .      50 

Amphibole   100 

Apatite   33,  107 

Aragonite 22,  34,  178 

Asphaltum 48 

Azurite   n,  34 

Beryl 18,  34,  104 

Beryllium  oxide 115 

Borax    50 

Calcite.i2,  22,30,  34, 122, 178 

Calcium    72 

Calcium  oxide 121 

Calcium  sulphate    120 

Carbon 46,  73,  91 

Carbonates 61,  62 

Carborundum 36,  43 

Celestite    33 

Cerium  oxide 112 

Cerussite   10,  34,  178 

Chalcocite 14 

Chromite 15 

Chromium  oxide 119 

Chrysocolla 1 1 

Cobalt  glass 54 

Cobalt  oxide 119 

Copper    72 

Copper  oxide 119 

Corundum I7>  34 

Covellite 14 

Cryolite   32,  34,  43 

Cyanine 33 

Cyanite 18,  34 

Diamond 46 

Diaspore 32 


PAGE. 

Didymium  nitrate 50 

Dolomite 12,  34 

Erbium  oxide 117 

Feldspar 99 

Fluorite 46 

Glass    45,  102 

Glass  (colored)  .  .55,  56,  103 

Gold 36,  53 

Gold  (colloidal) 53 

Graphite  (arc)  74 

Hematite    15,  119 

Iron  oxide   15,  119 

Kuntzite 42 

Lanthanum  nitrate  ....     50 

Lead  oxide 118 

Liquid  glass 50 

Magnesite    ......   13,  23,  31 

Magnesium  oxide  .  .102,  in 

Magnetite 15 

Malachite n,  34 

Manganous  oxide 118 

Mica   29 

Molybdenite 13,  41 

Neodymium  nitrate  ....     51 

Neodymium  oxide 118 

Nernst  glower 81,  126 

Nickel 53,  71 

Nickel  oxide 120 

Nitrocellulose 43 

Oligoclase 100,  129 

Orthoclase    99 

Osmium 90 

Phosphorus 45 

Platinum 119,  124 

Porcelain 102 

Potassium 77,  78 

Potassium  alum 50 

Potassium       permanga- 
nate       50 


PAGE. 

Pyrrhotite 14 

Quartz  (crystalline)  .    21,  34, 

36,  45,  140 

Quartz  (amorphous)   ...   21, 

34,  US 

Rhodochrosite   178 

Rutile 17,  34,  105 

Scheelite 16,  34 

Selenium 15 

Siderite 1 2,  34,  44,  178 

Silicates 66 

Silver 36,  52 

Smithsonite  .   10,  23,  34,  178 

Sodium 74,  77 

Sodium  silicate    19 

Sphalerite 57 

Spodumene  . .  19,  33,  34,  104 

Stannic  acid 119 

Stibnite 33 

Strontianite  .   n,  24,  34,  178 

Sulphates 62,  63 

Sulphuric  acid 50 

Tantalum 92 

Thorium  oxide 112 

Topaz 19,  34,  105 

Tricalcium  phosphate  . .   121 

Tungsten 92 

Uranium  oxide 112 

Vanadium  oxide    115 

Water   50 

Water  vapor 149 

Witherite  ...    n,  24,  34,  178 

Wollastonite 101 

Wulfenite 1 6,  34,  44 

Yttrium  chloride 51 

Yttrium  oxide    116 

Zincite    17,  118 

Zircon 16,  34 

Zirconium  oxide.    .   108,  in 

183 


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RB  17-60m-8,'60 
(B3395slO)4188 


General  Library 

University  of  California 

Berkeley 


